F.5 Exercise 12 (Light Rays) Suggested Answers
1. 3 m
2. (b) Slope = 1.5. The slope is the refractive index of glass.
(c) Wrong. It is because the light is travelling from air to glass. Total internal reflection will take place
only when the light is traveling from a denser medium into a less dense medium (for example from glass to
air) and the angle of incidence is greater than critical angle.
3. (a) convex lens
(b) virtual, erect
(c) (ii) Image distance = 24 cm, focal length = 12 cm
(d) The image size and image distance both decrease.
(e) A real image is formed in regions where the student cannot observe a clear image, e.g. the image is
formed in front of or behind the retina, or the image is formed between the near point and the eye.
4. (a) erect and virtual
(b) 12 cm from the lens
(c) 24 cm in front of the lens
MC
1-5
BDEAE
6-10
DDEBE
11-15 D E D D C
16-18 D B D
Explanations to selected mc
1. Speed of light in glass v = c / n = 3 x 108 / 1.5 = 2 x 108 m s-1.
Frequency is unchanged during refraction.
8. Only when the object is placed at the focus of the lens will the image coincide with the object. It is
because the refracted beam is parallel to the principal axis and after reflection from the plane mirror, the
parallel beam converges to the focus again.
9. Light ray (from LHS) parallel to the principal axis will be refracted as S.
Light ray (from LHS) passing through F will be refracted as R.
Light ray (from LHS) passing through like the figure will be refracted as Q.
Alternatively, imagine an object placed between the focus and the lens with the bottom at the
intersection point between the ray and the principal axis. Then construct a ray diagram to find the image.
Use the principle that ‘bottom of object to bottom of image’, work out the path of light ray.
11. Erect and diminished image produced must be due to a concave lens. Now the magnification is 1/3, so
the image distance / object distance = 1/3, the lens must therefore be at D.
12. A, B and C are impossible because the refracted rays are convergent but the lens is a divergent lens.
D is also not possible because only light ray parallel to the principal axis will be refracted from focus.
E is the only possible answer.
14. A diverging lens would move the image to the right.
A rectangular block of glass can also do the same thing.
15. The following combinations of lenses can give the desired effect (parallel incident beam becomes
parallel narrower beam)
A concave lens placed on LHS and a convex lens placed on RHS can also give parallel beam but must
be wider than before.