Class 10 · Physics · FBISE Islamabad

Chapter 3: Geometrical Optics

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Concept

1 3 mkConcept

State the laws of reflection of light.

LAW 1 The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

LAW 2 The angle of incidence is equal to the angle of reflection: i = r.

Answer i = r, and the incident ray, reflected ray and normal are coplanar.

Common mistake

Both laws must be stated. Writing only i = r loses the coplanarity mark.

4 4 mkConcept

What is total internal reflection? State the two conditions necessary for it.

DEFINE When light travelling in a denser medium strikes the boundary of a rarer medium at an angle greater than the critical angle, it is reflected entirely back into the denser medium.

CONDITION 1 Light must travel from a denser medium to a rarer medium.

CONDITION 2 The angle of incidence must be greater than the critical angle of the pair of media.

Answer Complete reflection back into the denser medium when i > critical angle.

Common mistake

This is the principle behind optical fibre — a common follow-up part worth an extra mark.

Numerical

2 4 mkNumerical

An object is placed 20 cm in front of a concave mirror of focal length 15 cm. Find the image distance.

p = 20 cmf = 15 cmFind q

STEP 1 1/p + 1/q = 1/f

STEP 2 1/20 + 1/q = 1/15

STEP 3 1/q = 1/15 − 1/20 = (4 − 3)/60 = 1/60

STEP 4 q = 60 cm

Answer q = 60 cm, real and on the same side as the object

Common mistake

A positive q for a concave mirror means the image is real and inverted. Say so — the scheme awards a mark for the nature of the image.

3 4 mkNumerical

Light travels from air into glass of refractive index 1.5 at an angle of incidence of 30°. Find the angle of refraction.

n = 1.5i = 30°Find r

STEP 1 n = sin i ÷ sin r

STEP 2 1.5 = sin 30° ÷ sin r

STEP 3 sin r = 0.5 ÷ 1.5 = 0.333

STEP 4 r = 19.5°

Answer r ≈ 19.5°

Common mistake

The ray bends towards the normal when it enters a denser medium, so r must come out smaller than i. Use that as a check.

This sheet was checked against the FBISE Islamabad 2024 marking scheme.