For Class 8, 9 & 10
Master the basics - and everything after gets easier
Concept-first questions with clear model answers in Physics, Chemistry, Maths and Biology, all NCERT-aligned. Start early, build the habit, and walk into your boards, NEET and JEE already ahead.
Interactive lessons
learn by playingDrag, slide and build - watch each concept come alive, then reveal the answer.
200 interactive lessons
Ohm's law
Class 10 Physics
Slide V & R, watch the bulb glow
Open →pH scale
Class 10 Chemistry
Slide across acids and bases
Open →Atomic number and mass number
Class 9 Chemistry
Add protons & neutrons, build shells
Open →Laws of reflection
Class 8 Physics
Change the angle, watch it bounce
Open →Volume of a sphere
Class 9 Maths
Grow the radius, see the volume
Open →Area of a trapezium
Class 8 Maths
Drag the sides, read the area
Open →Power of a lens
Class 10 Physics
Move the object, trace the rays
Open →Food chain and energy flow
Class 10 Biology
Follow the energy as it flows
Open →Speed
Class 8 Physics
Slide distance & time, watch the speed
Open →Density
Class 9 Physics
Pack mass into volume, float or sink
Open →Work done
Class 9 Physics
Push harder or farther, watch work grow
Open →Kinetic energy
Class 9 Physics
Speed it up - energy grows with the square
Open →Power of a lens
Class 10 Physics
Shorten the focal length, boost the power
Open →Mole concept
Class 9 Chemistry
Weigh out grams, count the moles
Open →Avogadro's number
Class 9 Chemistry
Add moles, count the particles
Open →Microscope magnification
Class 8 Biology
Grow the image, read the magnification
Open →Population density
Class 10 Biology
Add individuals, shrink the land, see crowding
Open →Simple interest
Class 8 Maths
Slide money, rate & time, watch interest
Open →Pythagoras theorem
Class 9 Maths
Stretch the two sides, get the hypotenuse
Open →Probability of an event
Class 10 Maths
Change the outcomes, watch the odds
Open →Newton's second law
Class 9 Physics
Push a mass, pick an acceleration
Open →Momentum
Class 9 Physics
Slide mass & velocity, build momentum
Open →Pressure
Class 8 Physics
Shrink the area, feel the pressure rise
Open →Weight
Class 9 Physics
Change the planet's gravity, watch your weight
Open →Refractive index
Class 10 Physics
Slow light in the medium, raise the index
Open →Resistors in series
Class 10 Physics
Add two resistors in a line
Open →Mass percentage of a solution
Class 9 Chemistry
Dissolve solute, read the strength
Open →Concentration of a solution
Class 9 Chemistry
Pack solute into less liquid
Open →Population change
Class 10 Biology
Balance births against deaths
Open →Compound microscope
Class 8 Biology
Combine eyepiece & objective lenses
Open →Area of a circle
Class 8 Maths
Grow the radius, watch the area square
Open →Volume of a cuboid
Class 8 Maths
Stretch length, breadth & height
Open →Electronic configuration and valency
Class 9 Chemistry
Slide the atomic number, build the atom
Open →Homologous series (alkanes)
Class 10 Chemistry
Add carbons, name the compound
Open →Mass number
Class 9 Chemistry
Add protons & neutrons, get the mass number
Open →Power
Class 9 Physics
More work in less time = more power
Open →Potential energy
Class 9 Physics
Lift a mass higher, store energy
Open →Wave speed
Class 9 Physics
Tune frequency & wavelength, set the speed
Open →Electric current
Class 10 Physics
Push charge per second, get the current
Open →Percentage
Class 8 Maths
Compare part to whole as a %
Open →Electron dot structure
Class 9 Chemistry
Draw valence electrons as dots
Open →Acceleration
Class 9 Physics
Speed up over time, find acceleration
Open →Distance, speed and time
Class 8 Physics
Set speed & time, cover the distance
Open →Frequency and time period
Class 9 Physics
Shorten the period, raise the frequency
Open →Heating effect of current
Class 10 Physics
Raise the current, watch heating soar
Open →Area of a triangle
Class 8 Maths
Set base & height, halve the rectangle
Open →Area of a rectangle
Class 8 Maths
Set length & breadth, fill the area
Open →Mean (average)
Class 9 Maths
Share the total equally across items
Open →Discount
Class 8 Maths
Slide price & % off, see the saving
Open →Heart rate
Class 10 Biology
Set heart rate & time, count the beats
Open →Resistors in parallel
Class 10 Physics
Wire two resistors side by side
Open →Electric charge
Class 10 Physics
Flow current over time, collect charge
Open →Electrical energy and units
Class 10 Physics
Run appliances, add up the units
Open →Kelvin temperature scale
Class 9 Chemistry
Slide Celsius, read the Kelvin
Open →Moles from number of particles
Class 9 Chemistry
Divide particles by Avogadro's number
Open →Ten percent law
Class 10 Biology
See 10% of energy reach the next level
Open →Area of a square
Class 8 Maths
Grow the side, square the area
Open →Volume of a cube
Class 8 Maths
Grow the edge, cube the volume
Open →Circumference of a circle
Class 8 Maths
Grow the radius, roll out the rim
Open →Surface area of a cube
Class 9 Maths
Grow the edge, cover six faces
Open →Potential difference
Class 10 Physics
Share work across charge, get volts
Open →Resistance from Ohm's law
Class 10 Physics
Divide voltage by current, get resistance
Open →Echo and SONAR
Class 9 Physics
Time the echo, find the distance
Open →Mass from moles
Class 9 Chemistry
Multiply moles by molar mass
Open →Breathing rate
Class 10 Biology
Set breathing rate & time
Open →Volume of a cylinder
Class 10 Maths
Set radius & height, fill the can
Open →Compound interest
Class 8 Maths
Compound money over years
Open →Profit and loss percentage
Class 8 Maths
Set cost & selling price, see profit %
Open →Perimeter of a rectangle
Class 8 Maths
Set length & breadth, walk the border
Open →Surface area of a sphere
Class 10 Maths
Grow the radius, wrap the ball
Open →Time period
Class 9 Physics
Raise the frequency, shrink the period
Open →Relative velocity
Class 9 Physics
Two objects approach - add their speeds
Open →Average velocity
Class 9 Physics
Average the start and end speeds
Open →Equations of motion (v = u + at)
Class 9 Physics
Accelerate from u for a time t
Open →Kelvin to Celsius
Class 9 Chemistry
Slide Kelvin, read the Celsius
Open →Population growth rate
Class 10 Biology
Balance births vs deaths per population
Open →Perimeter of a square
Class 8 Maths
Grow the side, walk four edges
Open →Perimeter of a triangle
Class 8 Maths
Add the three sides
Open →Area of a parallelogram
Class 8 Maths
Set base & height, slide the shape
Open →Area of a rhombus
Class 8 Maths
Set the two diagonals
Open →Equations of motion (distance)
Class 9 Physics
Start, accelerate, cover ground
Open →Joule's law of heating
Class 10 Physics
Raise current, resistance or time
Open →Electric power (P = VI)
Class 10 Physics
Multiply voltage by current
Open →Average atomic mass of isotopes
Class 9 Chemistry
Mix two isotopes by abundance
Open →Seed germination percentage
Class 9 Biology
Count sprouted seeds out of the total
Open →Volume of a cone
Class 9 Maths
Set radius & height, fill the cone
Open →Surface area of a cylinder
Class 9 Maths
Wrap the side and both ends
Open →Surface area of a cuboid
Class 9 Maths
Cover all six rectangular faces
Open →nth term of an AP
Class 10 Maths
Step from the first term by d
Open →Sum of an AP
Class 10 Maths
Add up the first n terms
Open →Focal length of a mirror
Class 10 Physics
Halve the radius to find the focus
Open →Speed of light in a medium
Class 10 Physics
Raise the index, slow the light
Open →Percentage purity
Class 9 Chemistry
Weigh the pure part of a sample
Open →Slope of a line
Class 10 Maths
Rise over run gives the steepness
Open →Percentage change
Class 8 Maths
Compare a new value to the old
Open →Volume of a hemisphere
Class 9 Maths
Grow the radius of half a ball
Open →Area of a sector
Class 10 Maths
Cut a slice of angle from a circle
Open →Length of an arc
Class 10 Maths
Measure the curved edge of a slice
Open →Slant height of a cone
Class 9 Maths
Combine radius & height for the slant
Open →Unit conversion (km/h to m/s)
Class 9 Physics
Convert km/h into m/s
Open →Equations of motion (v^2 = u^2 + 2as)
Class 9 Physics
Accelerate over a distance, find v
Open →Impulse
Class 9 Physics
Hit harder or longer, change momentum
Open →Wavelength
Class 9 Physics
Speed over frequency gives wavelength
Open →Oscillations
Class 9 Physics
Vibrate at a frequency for a time
Open →Cost of electricity
Class 10 Physics
Units times rate gives the bill
Open →Number of neutrons
Class 9 Chemistry
Take protons away from the mass number
Open →Curved surface area of a cone
Class 9 Maths
Wrap the slanted side of a cone
Open →Total surface area of a cone
Class 9 Maths
Add the base circle to the cone's side
Open →Curved surface area of a hemisphere
Class 9 Maths
Cover the dome of a hemisphere
Open →Diagonal of a square
Class 9 Maths
Cross a square corner to corner
Open →Unit conversion (m/s to km/h)
Class 9 Physics
Convert m/s into km/h
Open →Distance from velocities
Class 9 Physics
From two speeds, find the distance
Open →Diagonal of a rectangle
Class 9 Maths
Cross a rectangle corner to corner
Open →Diagonal of a cuboid
Class 9 Maths
The longest rod that fits in a box
Open →Area by Heron's formula
Class 9 Maths
Area from just the three sides
Open →Interior angle sum of a polygon
Class 8 Maths
Add up a polygon's inside angles
Open →Exterior angle of a regular polygon
Class 8 Maths
Share 360 among a polygon's corners
Open →Number of diagonals of a polygon
Class 8 Maths
Count the diagonals of a polygon
Open →Discriminant
Class 10 Maths
Test how many roots a quadratic has
Open →Sum of roots
Class 10 Maths
Sum of a quadratic's roots
Open →Punnett square (monohybrid cross)
Class 10 Biology
Cross two parents, predict the offspring
Open →Balancing chemical equations
Class 10 Chemistry
Slide coefficients until atoms balance
Open →Writing chemical formulae (valency)
Class 9 Chemistry
Criss-cross valencies into a formula
Open →Current from power
Class 10 Physics
Divide power by voltage for current
Open →Power (P = V^2 / R)
Class 10 Physics
Voltage squared over resistance
Open →Sine ratio
Class 10 Maths
Opposite over hypotenuse
Open →Cosine ratio
Class 10 Maths
Adjacent over hypotenuse
Open →Tangent ratio
Class 10 Maths
Opposite over adjacent
Open →Area of an equilateral triangle
Class 9 Maths
Area of an equilateral triangle
Open →Curved surface area of a cylinder
Class 9 Maths
Wrap only the curved side
Open →Loss percentage
Class 8 Maths
Sell below cost, find the loss %
Open →Amount with simple interest
Class 8 Maths
Principal plus its simple interest
Open →Distance formula
Class 10 Maths
Straight distance between two points
Open →States of matter
Class 9 Chemistry
Heat particles solid → liquid → gas
Open →Parts of a plant cell
Class 8 Biology
Tap a cell part to see its job
Open →Diagonal of a cube
Class 9 Maths
Longest diagonal through a cube
Open →Total surface area of a hemisphere
Class 9 Maths
Dome plus its flat circle
Open →Sum of first n natural numbers
Class 10 Maths
Add 1 + 2 + ... + n instantly
Open →Range of data
Class 9 Maths
Spread from smallest to largest
Open →Class mark
Class 9 Maths
Midpoint of a class interval
Open →Selling price from profit percent
Class 8 Maths
Mark up cost by a profit %
Open →Perimeter of a sector
Class 10 Maths
Two radii plus the curved arc
Open →Circumference from diameter
Class 8 Maths
Circumference straight from diameter
Open →Power (P = F x v)
Class 9 Physics
Force times velocity gives power
Open →Percentage of a number
Class 8 Maths
Find a percentage of a number
Open →Series and parallel circuits
Class 10 Physics
Break a bulb in series vs parallel
Open →Symbols of elements
Class 9 Chemistry
Match each element to its symbol
Open →Free fall (velocity)
Class 9 Physics
Drop from a height, hit this speed
Open →Free fall (time)
Class 9 Physics
How long a drop takes
Open →Free fall (distance)
Class 9 Physics
Distance fallen in a given time
Open →Complement of an event
Class 10 Maths
Chance an event does NOT happen
Open →Product of roots
Class 10 Maths
Product of a quadratic's roots
Open →Exterior angle theorem
Class 9 Maths
Exterior angle = sum of remote interiors
Open →Complementary angles
Class 10 Maths
What adds to 90 degrees
Open →Supplementary angles
Class 9 Maths
What adds to 180 degrees
Open →Perimeter of a semicircle
Class 10 Maths
Curved half plus the diameter
Open →Area of a semicircle
Class 10 Maths
Half the area of a circle
Open →Turning effect (moments)
Class 9 Physics
Balance the see-saw with moments
Open →Reflex arc
Class 10 Biology
Step through a reflex, stimulus to action
Open →Buoyant force (upthrust)
Class 9 Physics
Displace liquid, feel the upthrust
Open →Relative density
Class 9 Physics
Compare a density to water's
Open →Power in lifting a load
Class 9 Physics
Lift a load, faster needs more power
Open →Cosecant ratio
Class 10 Maths
Hypotenuse over opposite
Open →Secant ratio
Class 10 Maths
Hypotenuse over adjacent
Open →Cotangent ratio
Class 10 Maths
Adjacent over opposite
Open →Height from angle of elevation
Class 10 Maths
Height from an angle of elevation
Open →Area of a quadrant
Class 10 Maths
A quarter of a circle's area
Open →Interior angle of a regular polygon
Class 8 Maths
One inside angle of a regular polygon
Open →Sum of first n odd numbers
Class 10 Maths
Add the first n odd numbers
Open →Sum of first n even numbers
Class 10 Maths
Add the first n even numbers
Open →Quadratic formula (a root)
Class 10 Maths
Larger root of a quadratic
Open →LCM from HCF
Class 10 Maths
LCM from the product and HCF
Open →Depreciation
Class 8 Maths
Value drops by a % each year
Open →Cost price from selling price
Class 8 Maths
Work back to the cost price
Open →Downstream speed
Class 8 Maths
Row with the current
Open →Upstream speed
Class 8 Maths
Row against the current
Open →Average speed for a round trip
Class 8 Maths
Average speed there and back
Open →Sales tax / GST
Class 8 Maths
Tax added on a price
Open →Area of a ring (annulus)
Class 10 Maths
Area of a ring between two circles
Open →Edge of a cube from volume
Class 9 Maths
Edge back from the volume
Open →Radius from area
Class 10 Maths
Radius back from a circle's area
Open →Side from area of a square
Class 8 Maths
Side back from a square's area
Open →Height of a triangle from area
Class 9 Maths
Height back from area and base
Open →Rate from simple interest
Class 8 Maths
Rate back from the interest
Open →Time from simple interest
Class 8 Maths
Time back from the interest
Open →Principal from simple interest
Class 8 Maths
Principal back from the interest
Open →Mean proportional
Class 10 Maths
Geometric mean of two numbers
Open →Fourth proportional
Class 8 Maths
Complete the proportion a : b = c : ?
Open →Marked price from selling price
Class 8 Maths
Marked price back from the sale price
Open →Chambers of the human heart
Class 10 Biology
Tap a heart chamber to see its job
Open →Equation of a line (y = mx + c)
Class 9 Maths
Read y off a straight line
Open →Average term of an AP
Class 10 Maths
Average of first and last term
Open →Number of terms in an AP
Class 10 Maths
How many terms in an AP
Open →Midpoint of two points
Class 10 Maths
x-coordinate of a midpoint
Open →Empirical mode
Class 10 Maths
Estimate the mode from mean & median
Open →Length of a shadow
Class 10 Maths
Shadow from height and sun angle
Open →Train crossing a pole
Class 8 Maths
Speed to cross a pole
Open →Time and work
Class 8 Maths
More workers, fewer days
Open →Dividing in a ratio
Class 8 Maths
Split a total in a ratio
Open →Unitary method
Class 8 Maths
Cost of a single item
Open →Your progress — Foundation - Class 8 to 10
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Showing 14 questions in Physics for Class 10. Tap a card to reveal the answer.
PhysicsLight - Reflection and RefractioneasyWhy does a ray of light bend when it passes from air into glass?
Reveal answer ↓
What it is
Light bends (refracts) when its speed changes on moving between media of different densities.
Answer
Light bends because its speed changes when it moves from one medium to another. Glass is optically denser than air, so light slows down on entering glass and bends towards the normal. This bending of light due to a change in speed is called refraction.
n = speed in vacuum / speed in medium = c/v
- •Speed of light changes between media
- •Denser medium -> bends towards the normal
- •Governed by refractive index n = c/v
Why learn this
It's why lenses, spectacles, cameras and your own eye can focus - the basis of all optics.
💡 Memory trick
Enter a denser medium -> slow down -> bend TOWARDS the normal (like a car hitting mud).
PhysicsLight - Reflection and RefractionmediumState the mirror formula and the sign convention used for a concave mirror.
Reveal answer ↓
What it is
The mirror formula links focal length, object distance and image distance for a curved mirror.
Answer
The mirror formula relates focal length f, image distance v and object distance u as 1/f = 1/v + 1/u. By the New Cartesian sign convention, distances are measured from the pole; distances measured against the incident light (to the left) are negative, so for a concave mirror f and u are negative, and a real image distance v is also negative.
1/f = 1/v + 1/u ; m = -v/u
- •1/f = 1/v + 1/u
- •Distances measured from the pole (P)
- •Concave mirror focal length is negative
- •Magnification m = -v/u = h'/h
Why learn this
It's how we design car mirrors, telescopes, torches and a dentist's mirror.
💡 Memory trick
1/f = 1/v + 1/u. In New Cartesian signs, left-of-mirror distances are negative.
PhysicsLight - Reflection and RefractionmediumA convex lens has a focal length of 25 cm. Find its power.
Reveal answer ↓
What it is
The power of a lens tells how strongly it bends light; it is the reciprocal of its focal length in metres.
Object at 24 cm → image at 17.1 cm · diminished, inverted, real (m = -0.71)
Answer
Power P = 1 / f, where f is the focal length in metres. Here f = 25 cm = 0.25 m, so P = 1 / 0.25 = +4 dioptres (D). The power is positive because it is a convex (converging) lens.
P = 1 / f (dioptre), f in metres
- •P = 1 / f (f in metres)
- •f = 0.25 m
- •P = +4 D (positive for a convex lens)
Why learn this
It's how opticians prescribe your spectacles - the number on the prescription is the power.
💡 Memory trick
Power = 1 / focal length (in metres). Shorter focal length = stronger (higher power) lens.
PhysicsLight - Reflection and RefractionmediumA convex lens has a focal length of 0.5 m. Find its power. Slide the focal length to explore.
Reveal answer ↓
What it is
The power of a lens measures how strongly it converges or diverges light.
Answer
Power = 1 / focal length (in metres) = 1 / 0.5 = +2 dioptre (D). A convex (converging) lens has positive power; the shorter its focal length, the higher its power.
P = 1 / f
- •Power P = 1 / f (f in metres)
- •Unit: dioptre (D)
- •Convex lens: positive power; shorter f -> more power
Why learn this
It's the number written on your spectacle prescription.
💡 Memory trick
Power = 1 / focal length (in metres). Shorter focal length -> more power.
PhysicsLight - Reflection and RefractionmediumLight slows to 2 x 10^8 m/s in a medium (c = 3 x 10^8 m/s). Find the refractive index. Slide to explore.
Reveal answer ↓
What it is
The refractive index of a medium is how many times slower light travels in it than in vacuum.
Answer
Refractive index n = speed in vacuum / speed in medium = (3 x 10^8) / (2 x 10^8) = 1.5. A larger refractive index means light travels more slowly in the medium and bends more on entering it.
n = c / v
- •n = c / v
- •It has no unit (a ratio)
- •Slower light in medium -> higher n; glass is about 1.5
Why learn this
It sets how much light bends and is the key number for lenses and prisms.
💡 Memory trick
n = c / v. Slower light in the medium -> higher n.
PhysicsLight - Reflection and RefractioneasyA spherical mirror has a radius of curvature of 40 cm. Find its focal length. Slide to explore.
Reveal answer ↓
What it is
The focal length of a spherical mirror is half its radius of curvature.
Answer
Focal length = radius of curvature / 2 = 40 / 2 = 20 cm. The principal focus of a spherical mirror lies exactly midway between the pole and the centre of curvature.
f = R / 2
- •f = R / 2
- •Focus is midway to the centre of curvature
- •Unit: cm
Why learn this
It's how we work out where a mirror focuses light for torches and telescopes.
💡 Memory trick
f = R / 2. The focus sits halfway to the centre.
PhysicsLight - Reflection and RefractionmediumHow fast does light travel in glass (n = 1.5), given c = 3 x 10^8 m/s? Slide to explore.
Reveal answer ↓
What it is
The speed of light in a medium is the speed in vacuum divided by the medium's refractive index.
Answer
Speed in medium = c / n = (3 x 10^8) / 1.5 = 2 x 10^8 m/s. The larger the refractive index, the more the light slows down, and the more it bends on entering the medium.
v = c / n
- •v = c / n
- •c = 3 x 10^8 m/s in vacuum
- •Higher refractive index -> slower light
Why learn this
This slowing of light is exactly what makes lenses and prisms bend it.
💡 Memory trick
v = c / n. Higher index -> slower light.
PhysicsLight - Reflection and RefractioneasyList the characteristics of an image formed by a plane mirror.
Reveal answer ↓
What it is
A plane mirror forms a virtual, erect image of the same size, as far behind the mirror as the object is in front, and laterally inverted.
Answer
The image formed by a plane mirror is virtual (cannot be caught on a screen) and erect (upright). It is of the same size as the object. It is formed as far behind the mirror as the object is in front of it. The image is laterally inverted, meaning the left and right sides appear interchanged, which is why the word AMBULANCE is written in a mirror-reversed form on vehicles.
- •Virtual and erect
- •Same size as the object
- •Image distance = object distance (behind mirror)
- •Laterally inverted
Why learn this
It explains why AMBULANCE is written in reverse on the front of the vehicle.
💡 Memory trick
Plane mirror image: Virtual, Erect, Same size, Laterally inverted (V-E-S-L).
PhysicsLight - Reflection and RefractionmediumState one use each of a concave and a convex mirror, with a reason.
Reveal answer ↓
What it is
Concave mirrors converge light and can form real or virtual images; convex mirrors diverge light and always form small, virtual, erect images.
Answer
A concave mirror is used as a shaving or make-up mirror because, when the face is held close (within the focus), it forms a magnified, erect, virtual image; it is also used in torches and headlights to produce a parallel beam. A convex mirror is used as a rear-view mirror in vehicles because it always forms a small, erect, virtual image and gives a wide field of view, letting the driver see more traffic behind.
- •Concave: converging, real or virtual images
- •Concave used as shaving mirror, in headlights
- •Convex: diverging, always small virtual erect image
- •Convex used as vehicle rear-view mirror (wide view)
Why learn this
It is why concave mirrors are used as shaving/dentist mirrors and convex mirrors as vehicle rear-view mirrors.
💡 Memory trick
Concave = caves in, converging (real images possible). Convex = bulges out, diverging (always small virtual).
PhysicsLight - Reflection and RefractionhardAn object is placed 30 cm from a concave mirror of focal length 20 cm. Find the image distance.
Reveal answer ↓
What it is
The mirror formula 1/v + 1/u = 1/f relates image distance, object distance and focal length.
Answer
For a concave mirror, f = -20 cm and the object distance u = -30 cm (New Cartesian convention). Using 1/v + 1/u = 1/f: 1/v = 1/f - 1/u = 1/(-20) - 1/(-30) = -1/20 + 1/30. Taking LCM 60: = -3/60 + 2/60 = -1/60. So v = -60 cm. The image is real, inverted and formed 60 cm in front of the mirror.
1/v + 1/u = 1/f ; m = -v/u
- •1/v + 1/u = 1/f
- •f = -20, u = -30 (signs)
- •1/v = -1/20 + 1/30 = -1/60
- •v = -60 cm (real, inverted image)
Why learn this
It lets us calculate exactly where an image forms and its size.
💡 Memory trick
1/f = 1/v + 1/u. Use the New Cartesian sign convention (distances left of mirror are negative).
PhysicsLight - Reflection and RefractionmediumDefine refractive index. What does a refractive index of 1.5 for glass mean?
Reveal answer ↓
What it is
Refraction is the bending of light when it changes speed passing between media; the refractive index measures how much the speed changes.
Answer
The refractive index of a medium is the ratio of the speed of light in vacuum (or air) to the speed of light in that medium, that is n = c/v. A refractive index of 1.5 for glass means that light travels 1.5 times faster in vacuum than in glass, or equivalently that the speed of light in glass is 1/1.5 of its speed in vacuum. A higher refractive index means the medium is optically denser and bends light more.
n = speed of light in vacuum / speed in medium = c/v
- •Refraction: bending due to change of speed
- •Refractive index n = c/v
- •n = 1.5 means light is 1.5 times slower in glass
- •Higher n -> optically denser -> bends more
Why learn this
It is why a pencil looks bent in water and how lenses focus light.
💡 Memory trick
Into a denser medium -> slows down -> bends towards the normal. n = c/v.
PhysicsLight - Reflection and RefractionmediumDifferentiate between a convex and a concave lens and give one use of each.
Reveal answer ↓
What it is
A convex lens converges light and can form real or virtual images; a concave lens diverges light and always forms a small, virtual, erect image.
Answer
A convex (converging) lens is thicker in the middle and bends parallel rays of light to meet at a focus; it can form real, inverted images or a magnified virtual image, and is used in magnifying glasses, cameras and to correct hypermetropia. A concave (diverging) lens is thinner in the middle and spreads parallel rays apart; it always forms a small, erect, virtual image and is used to correct myopia (short-sightedness).
- •Convex: converging, thick middle
- •Convex forms real or virtual images
- •Concave: diverging, thin middle
- •Concave always forms small virtual image
Why learn this
Convex lenses are used in magnifiers and cameras; concave lenses correct short-sight.
💡 Memory trick
Convex = converging (thick middle); Concave = diverging (thin middle).
PhysicsLight - Reflection and RefractionhardAn object is placed 15 cm from a convex lens of focal length 10 cm. Find the image distance.
Reveal answer ↓
What it is
The lens formula 1/v - 1/u = 1/f relates image distance, object distance and focal length of a lens.
Answer
For a convex lens, f = +10 cm and the object distance u = -15 cm. Using 1/v - 1/u = 1/f: 1/v = 1/f + 1/u = 1/10 + 1/(-15) = 1/10 - 1/15. Taking LCM 30: = 3/30 - 2/30 = 1/30. So v = +30 cm. The image is real, inverted and formed 30 cm on the other side of the lens.
1/v - 1/u = 1/f ; m = v/u
- •1/v - 1/u = 1/f
- •f = +10, u = -15
- •1/v = 1/10 - 1/15 = 1/30
- •v = +30 cm (real, inverted)
Why learn this
It predicts the position and size of an image formed by a lens.
💡 Memory trick
Lens: 1/v - 1/u = 1/f (note the minus, unlike a mirror). Magnification m = v/u.
PhysicsLight - Reflection and RefractionmediumFind the power of a convex lens of focal length 25 cm.
Reveal answer ↓
What it is
The power of a lens is the reciprocal of its focal length in metres, measured in dioptres.
Answer
The power of a lens P = 1/f, where f is the focal length in metres. Here f = 25 cm = 0.25 m, and the lens is convex so f is positive. P = 1/0.25 = +4 D. So the power of the lens is +4 dioptres. A concave lens would have a negative power.
Power P = 1/f (metres), unit dioptre
- •P = 1/f (f in metres)
- •f = 25 cm = 0.25 m
- •P = 1/0.25 = +4 D
- •Unit is the dioptre (D)
Why learn this
Opticians prescribe spectacle lenses in terms of their power.
💡 Memory trick
Power P = 1/f (in metres), unit dioptre (D). Convex power is +, concave is -.
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