Engineering Physics (ESC1002)
Self Practice: Waves & Optics 3.1 – Thin Lens Light Ray Diagrams
Section A: Structured Questions
1
The diagram shows an object and its virtual image formed by a convex lens.
(a)
Draw two rays that originates from the object to show how the image is formed on
the diagram. In the diagram, mark the position of the principal focus, F.
[3]
Solution:
- correct two ray diagrams [2]
- indicate the position of the focal points on both sides, F
[1]
1
(b)
Name one instrument which makes use of the set-up as in 6(a).
[1]
Solution:
A magnifying glass [1]
……………………………………………………………………………………………….
(c)
Name another instrument, which uses a convex lens to produce a real and
magnified image.
[1]
Solution:
A slide projector [1]
……………………………………………………………………………………………….
2
2
Fig. 2.1 shows a convex lens forming a real image of a point object at the opposite side
of the lens.
Fig. 2.1
(a)
What should we do to clarify the image formed is real?
Solution:
We can place a screen at the position of the image.
Only real images can be formed on the screen.
(b)
[2]
[1]
[1]
Explain why the light rays near the two ends of the lens bend more when
compared to those near the centre of the lens.
[3]
Solution:
By Snell’s law, the ratio of the sine of the angle of incidence to that of refraction is
constant for a certain material.
[1]
Thus, if the angle of incidence increases, the amount of deviation of light passing
through the lens also increases. [1]
Light rays are incident on the surface of the convex lens at different angles and
near the ends of the lens, the angles of incidence of the light rays are larger than
those near the centre of the lens. [1]
……………………………………………………………………………………………….
3
(c)
Describe what happens when light is passing through the surface of the lens (the
air-glass interface and the glass-air interface).
[2]
Solution:
Refraction occurs at the surface of the lens. As light rays travel slower in glass
than in air, the light rays that pass through the air-glass interface bend towards
the normal. [1]
But they bend away from the normal when passing through the glass-air interface
since light travels faster in air than in glass. [1]
……………………………………………………………………………………………….
4
3
Fig. 3.1 shows an object placed being at a distance in front of a thin converging lens
with optical centre C and focal length F.
object
2F
F
F
2F
Fig. 3.1
(a)
Draw the ray diagram on Fig. 3.1 to show how the image is formed on the
diagram.
[6]
object
2F
F
F
2F image
5
Solution:
- light ray (i) drawn correctly – cuts F on the other side of the lens [1]
- light ray (ii) drawn correctly – passes through the optical centre, C [1]
- correct arrow directions on light ray (i) and (ii) [1]
- same size [1]
- inverted image drawn [1]
- image drawn at position 2F [1]
(b)
State two characteristics of the image.
[2]
Solution:
The image is inverted/real/the same size. [2]
……………………………………………………………………………………………….
(c)
Name one instrument which makes use of the set-up as in Fig. 3.1.
[1]
Solution:
The photocopier making same-sized copy. [1]
……………………………………………………………………………………………….
6