NCERT Solutions: Light — Reflection and Refraction (Class 10 Science)

Chapter overview

This chapter covers how light behaves at mirrors (reflection) and at the boundary between two transparent media (refraction), and builds up to the working of lenses and the human eye.

Key concepts and laws

• Laws of reflection: the angle of incidence equals the angle of reflection, and the incident ray, reflected ray, and normal all lie in the same plane.
• Mirror formula: 1/v + 1/u = 1/f, where u is object distance, v is image distance, and f is focal length (sign convention: distances measured from the pole, with distances against the direction of incident light taken as negative).
• Magnification (mirrors): m = -v/u = h'/h
• Laws of refraction: the incident ray, refracted ray, and normal lie in the same plane, and sin i / sin r is a constant for a given pair of media (Snell’s Law).
• Lens formula: 1/v - 1/u = 1/f
• Lens power: P = 1/f (in metres), measured in dioptres (D).

Worked practice problem 1 — Concave mirror

Question: An object 4 cm tall is placed 15 cm in front of a concave mirror of focal length 10 cm. Find the position, size, and nature of the image.

Solution: Using the mirror formula with u = -15 cm and f = -10 cm:

1/v = 1/f - 1/u = -1/10 - (-1/15) = -1/10 + 1/15 = -3/30 + 2/30 = -1/30
v = -30 cm

Magnification: m = -v/u = -(-30)/(-15) = -2

Image height: h' = m × h = -2 × 4 = -8 cm

Answer: The image forms 30 cm in front of the mirror, is 8 cm tall, real, inverted, and magnified.

Worked practice problem 2 — Convex lens

Question: A convex lens of focal length 20 cm forms an image of an object placed 30 cm from the lens. Find the image distance and magnification.

Solution: Using the lens formula with u = -30 cm and f = 20 cm:

1/v = 1/f + 1/u = 1/20 + (-1/30) = 3/60 - 2/60 = 1/60
v = 60 cm

Magnification: m = v/u = 60/(-30) = -2

Answer: The image forms 60 cm on the other side of the lens, is real, inverted, and twice the size of the object.

Common mistakes to avoid

• Mixing up the sign convention between mirrors and lenses — they’re not the same.
• Forgetting that a virtual image has a positive v for mirrors but the object distance u is always negative when the object is in front of the mirror/lens.
• Confusing convex mirror (always virtual, diminished image) with convex lens (varies with object position) behavior.

Quick revision table

Mirror/Lens type	Object position	Image nature
Concave mirror	Beyond C	Real, inverted, diminished
Concave mirror	At F	Real, inverted, highly magnified, at infinity
Concave mirror	Between P and F	Virtual, erect, magnified
Convex lens	Beyond 2F	Real, inverted, diminished
Convex lens	At F	Real, inverted, highly magnified, at infinity
Convex lens	Between F and lens	Virtual, erect, magnified

Looking for the official textbook PDF for this chapter? Visit our Class 10 NCERT Books page for the direct download link.