For Class 8, 9 & 10
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Showing 3 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.
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.