Rectilinear Propagation of Light
Senior 1 · Physics
Rectilinear Propagation of Light
Lesson 3 of 5
23 min
On a bright morning, lie in a dark room whose only light comes through a small hole in a curtain or a nail hole in the iron-sheet door. On the wall opposite the hole you may see a faint, coloured picture of the world outside: the trees, the road, a passing moto, all upside down. No lens and no screen were made for it; a small hole and straight-line light are enough. People have known this for over two thousand years, and the Latin name for such a dark room, camera obscura, gave us the word "camera".
Figure 3.1: How a pinhole camera forms an image. Rays from the top of the object pass through the hole to the bottom of the screen, and rays from the bottom go to the top.
Light from each point of the object travels in a straight line, and only a narrow pencil of rays from each point gets through the tiny hole. Rays from the top of the object therefore land at the bottom of the screen, and rays from the left land on the right. So the image is inverted (upside down) and also turned left to right. It is a real image: the light really arrives there, and it can be caught on a screen.
Definition
A pinhole camera is a light-tight box with a very small hole at one end and a translucent screen at the other. It forms a real, inverted image of an object because light travels in straight lines.
The magnification of the image is how many times taller the image is than the object:
where is the image height, the object height, the distance from the pinhole to the screen (the image distance) and the distance from the object to the pinhole (the object distance). has no unit. Example: a camera long pointed at a candle away gives .
In Figure 3.1 the ray from the top of the object and the ray from its bottom cross at the pinhole. They form two triangles with the same angles, one on each side of the hole: a big one with the object as its base and height , and a small one with the image as its base and height . Triangles with equal angles are similar, so their sides are in the same ratio:
Worked example 1: the size of the image
A candle flame tall is in front of a pinhole camera whose screen is behind the hole. Find the magnification and the height of the image.
The image is tall, upside down.
Worked example 2: how tall is the tree?
A pinhole camera long forms an image tall of a tree away. How tall is the tree?
Use the same unit for and : .
Worked example 3: how long a box?
Uwase wants the image of a building tall, standing away, to be tall. How far behind the pinhole must the screen be?
Figure 3.2: Why the hole must be small. Each point of the object makes a patch of light the shape of the hole; a big hole makes big patches that overlap and blur the picture.
| Change | Effect on the image | Reason |
|---|---|---|
| move the screen further from the hole (longer box) | larger, but fainter | increases, so increases; the same light is spread over a bigger area |
| move the object closer | larger | decreases, so increases |
| make the hole larger | brighter but blurred | more light enters, but each point of the object makes a bigger patch (Figure 3.2) |
| make a second hole | two images, side by side | each hole forms its own image |
| put a convex lens in a large hole | bright AND sharp | the lens brings the rays from each point back together at one point: this is how real cameras and your eye work |
Important
A safe way to watch the Sun. A pinhole camera never points your eye at the Sun: you look only at the image on the screen, with your back to the Sun. During a partial eclipse, even the gaps between the leaves of a tree act as pinholes, and the ground beneath it is covered in tiny crescent-shaped images of the Sun.
Warning
Common mistakes. (1) Writing : it is image over object, . (2) Mixing units, such as in metres and in centimetres: change both to the same unit first. (3) Calling the pinhole image virtual: it is REAL, because it forms on the screen.
Tip
The rays cross at the hole, so the picture arrives upside down and back to front. For the size, remember "image over object equals v over u": everything on the IMAGE side goes on top.
Light enters your eye through a small opening, the pupil, and a lens behind it forms a real, inverted image on the retina at the back of the eye. Your brain learns to read the upside-down picture the right way up. Phone cameras work the same way, with a lens and a light sensor in place of the tracing paper.
Explaining the pinhole camera and introducing ray diagrams (Physics with Simon Poliakoff). Watch how two rays from the top and bottom of the object are enough to show why the image is inverted.
Practice
Key points
Sources and credits
Written by Ganzaa: the lesson text, the activity, the ray diagrams, the worked examples and the practice exercises.
