These eight original questions cover the lessons in SPM Physics light and optics. Draw a diagram and write your full working before opening each answer.
Questions
Question 1. Name the three rays used to locate an image formed by a convex lens.
Answer
A ray parallel to the axis, which passes through the far-side focal point. A ray through the optical centre, which goes straight on. A ray through the near-side focal point, which emerges parallel to the axis.
Question 2. A convex lens has f = 8 cm. An object is 24 cm from the lens. Find v and the magnification.
Answer
1/v = 1/8 − 1/24 = 3/24 − 1/24 = 2/24 = 1/12, so v = 12 cm.
m = 12 ÷ 24 = 0.50. The image is real, inverted and diminished.
Question 3. A convex lens has f = 20 cm and an object is 15 cm from it. Find v and m, and describe the image.
Answer
1/v = 1/20 − 1/15 = 3/60 − 4/60 = −1/60, so v = −60 cm.
m = 60 ÷ 15 = 4. The negative sign means a virtual image, which is upright and magnified, on the same side as the object.
Question 4. Light enters glass from air with an angle of incidence of 45° and an angle of refraction of 28°. Calculate the refractive index.
Answer
n = sin 45° ÷ sin 28° = 0.707 ÷ 0.469 = 1.51.
Question 5. Calculate the critical angle of a plastic with n = 1.60.
Answer
sin c = 1 ÷ 1.60 = 0.625, so c = 38.7°.
Question 6. Light in air strikes a glass block at an angle of incidence of 80°. A student says it is totally reflected. Is the student correct?
Answer
No. The light is going from air, the less dense medium, into glass, the denser medium, so the direction condition fails. The light refracts into the glass.
Total internal reflection needs light travelling from the denser to the less dense medium. See explaining total internal reflection.
Question 7. Explain why an optical fibre carries light along its length without leaking through the sides.
Answer
Light travels in the glass core, which has a higher refractive index than the cladding around it. The light strikes the boundary at angles greater than the critical angle.
It is totally reflected each time and stays within the core along the length of the fibre.
Question 8. State where the object is placed for a magnifying glass and describe the final image.
Answer
The object is inside the focal length of the convex lens. The final image is virtual, upright and magnified, on the same side as the object.
If you got these wrong
For Questions 2 and 3, revisit calculating image position and magnification. Questions 4 to 7 link to applying refraction and refractive index and the total internal reflection lesson.
The timed original practice session builder can turn this set into a timed session. For a teacher to work through the questions you missed, see online one-to-one Physics tuition.