Fig. 30.1 shows a simple pinhole camera made from a cardboard box. Light from a candle enters through a small pinhole at the front and forms an image on a t...
Question 1 Report
Fig. 30.1 shows a simple pinhole camera made from a cardboard box. Light from a candle enters through a small pinhole at the front and forms an image on a translucent screen at the back. The candle is 20 cm tall and is placed 60 cm from the pinhole. The box is 15 cm long. A student uses this camera as part of a class project on the properties of light and image formation. She traces the image on the screen and measures its height to verify her calculation.
(a) State whether the image on the screen is upright or inverted. [1]
(b) Explain why the image is inverted. [1]
(c) Calculate the height of the image using similar triangles (image height / object height = image distance / object distance). [2]
(d) State and explain what would happen to the image if the pinhole were made larger. [2]
(e) State one similarity between the pinhole camera and the human eye. [1]
Answer Details
Marking Scheme
- (a) [1 mark]: The image is inverted.
- (b) [1 mark]: Light travels in straight lines through the pinhole, so rays from the top of the candle travel downward through the pinhole to the bottom of the screen, and rays from the bottom travel upward to the top of the screen.
- (c) [2 marks]: \(\dfrac{\text{image height}}{20} = \dfrac{15}{60}\) [1]. Image height = \(20 \times \dfrac{15}{60} = 5.0\) cm [1].
- (d) [2 marks]: The image becomes brighter because more light enters through the larger hole [1]. The image also becomes blurred (less sharp) because light from each point on the object can pass through a wider opening, spreading over a larger area on the screen [1].
- (e) [1 mark]: Any one valid similarity, e.g.: both form inverted images; both have a small aperture (pupil/pinhole); both detect light on a surface at the back (retina/screen).
Explanation
A pinhole camera works because light travels in straight lines. Each point on the object sends out light in all directions, but the tiny pinhole selects only a narrow cone of rays. A ray from the top of the candle passes through the pinhole and continues in a straight line to the bottom of the screen; a ray from the bottom of the candle ends up at the top. This crossing of rays produces an inverted image.
The image size is found using similar triangles. The triangle formed by the object and the pinhole is similar to the triangle formed by the image and the pinhole:
\(\dfrac{\text{image height}}{\text{object height}} = \dfrac{\text{image distance (box length)}}{\text{object distance}}\)
\(\dfrac{h_i}{20} = \dfrac{15}{60} = 0.25\)
\(h_i = 20 \times 0.25 = 5.0\) cm
Making the pinhole larger allows more light through (brighter image) but destroys sharpness. With a larger hole, multiple rays from the same point on the object can enter at slightly different angles, each hitting a different spot on the screen. The point becomes a disc of confusion, and the image blurs. The pinhole camera trades brightness for sharpness.
The human eye works on the same principle: light enters through a small aperture (the pupil) and forms an inverted image on the retina at the back. The eye also has a lens to focus the light more precisely, which a basic pinhole camera lacks.
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