Light sources and how we see
A light source is anything that gives out its own light. Examples: the Sun, stars, a candle flame, a bulb, a phone screen. These are luminous objects.
Most things, like a book, a tree or the Moon, make no light. They are non-luminous. We see them only because light from a source falls on them and bounces into our eyes.
So seeing is a chain: source → object → eye. In a fully dark room you cannot see a book, because no light comes from it.
Light travels in straight lines
Look through three cards with holes. You see a candle only when all three holes are in one straight line. Shift one card and the light is blocked. This shows that light travels in straight lines. Scientists call this the rectilinear propagation of light (rectilinear = in a straight line).
We draw this with a model: a ray is a straight line with an arrow showing the direction light moves. A beam is a bundle of many rays. Rays are not real objects; they are a drawing tool that helps us predict shadows and images.
How fast is light?
In empty space (vacuum) light travels about 300 000 km every second (3 × 108 m/s). Sunlight takes about 8 minutes 20 seconds to reach Earth. Light is much faster than sound, which is why you see lightning before you hear thunder.
Transparent, translucent and opaque
- Transparent: lets almost all light pass; you see clearly through it (clean glass, clear water, air).
- Translucent: lets some light pass but scatters it; you see only a blur (butter paper, frosted glass, thin cloth).
- Opaque: lets no light pass (wood, metal, a brick, your hand).
When light hits something, three things can happen: it can pass through (transmission), bounce back (reflection) or be soaked up (absorption, which warms the object). White light is a mix of colours. A green leaf reflects green and absorbs the others; a black T-shirt absorbs nearly all colours, so it gets hot in the sun. Light also bends when it goes from air into water or glass (refraction); mirrors and lenses use reflection and refraction to make images. These have their own lessons.
Shadows, umbra, penumbra and eclipses
A shadow forms when an opaque object blocks light. You need a light source, an opaque object and a surface (screen). Because light goes straight, the shadow has the same outline as the object.
With a tiny (point) source the shadow is sharp. With a wide source the shadow has two parts: the umbra, the dark middle that gets no light, and the penumbra, the lighter edge that gets light from part of the source.
Shadow size: moving the object closer to the lamp makes the shadow bigger. By similar triangles, shadow size ÷ object size = (lamp-to-screen distance) ÷ (lamp-to-object distance).
Eclipses
A solar eclipse happens when the Moon comes between the Sun and the Earth and its shadow falls on Earth. A lunar eclipse happens when the Earth comes between the Sun and the Moon and Earth's shadow falls on the Moon. Inside the umbra the eclipse is total; inside the penumbra it is partial. Never look at the Sun directly, even in an eclipse.
The pinhole camera
A pinhole camera is a light-proof box with a tiny hole at the front and a thin paper screen at the back. Rays from the top of an object travel straight through the hole and land at the bottom of the screen; rays from the bottom land at the top. So the image is inverted (upside down) and also flipped left to right.
- Bigger hole → brighter but blurrier image.
- Longer box → bigger image.
- Image height ÷ object height = box length ÷ object distance.
Try it at home
Shadow ruler: in a dark room, shine a phone torch on a wall. Hold a coin 20 cm from the torch, then 40 cm. Measure the shadow each time. Predict first: will the shadow grow or shrink when you move the coin closer to the torch? Then check with the slider in the last 3D step.
Pinhole viewer: poke a pin hole in one end of a shoebox and tape butter paper over a window cut in the other end. Cover your head and the box with a dark cloth and point the hole at a bright window. You will see the window upside down.
Key formulas and definitions
- Light travels in straight lines (rectilinear propagation)
- Speed of light in vacuum c ≈ 3 × 10⁸ m/s = 300 000 km/s
- Distance = speed × time (for light travel)
- Shadow size ÷ object size = lamp-to-screen distance ÷ lamp-to-object distance
- Pinhole camera: image height ÷ object height = box length ÷ object distance
- Umbra = full shadow; penumbra = part shadow
Worked examples
1. Sort these as luminous or non-luminous: the Sun, the Moon, a burning candle, a mirror, a firefly.
Luminous (make their own light): the Sun, a burning candle, a firefly. Non-luminous: the Moon and the mirror; they only reflect light that falls on them.
2. Sunlight takes about 500 s to reach Earth. Speed of light = 3 × 10⁸ m/s. How far away is the Sun?
Distance = speed × time = 3 × 10⁸ × 500 = 1.5 × 10¹¹ m = 150 million km.
3. A 6 cm tall toy stands 20 cm from a small bulb. The wall is 80 cm from the bulb. How tall is the shadow?
Shadow ÷ object = 80 ÷ 20 = 4. Shadow = 4 × 6 cm = 24 cm.
4. A pinhole camera is 15 cm long. A tree 10 m tall is 50 m away. How tall is the image?
Image ÷ object = box length ÷ distance = 0.15 m ÷ 50 m = 0.003. Image = 0.003 × 10 m = 0.03 m = 3 cm, upside down.
5. You see lightning and hear the thunder 3 s later. Sound travels 340 m/s. How far away was the lightning, and why can we ignore the light's travel time?
Distance ≈ 340 × 3 = 1020 m, about 1 km. Light covers 1 km in about 0.000003 s, which is far too small to notice.
6. Why does a wide fluorescent tube give a shadow with blurry edges, but a tiny LED gives a sharp one?
From a wide source, some points on the edge of the shadow receive light from only part of the tube. That part-lit region is the penumbra, so the edge looks blurry. A tiny LED is almost a point source, so there is no penumbra and the edge is sharp.
Common mistakes
- Thinking the Moon is a light source. It only reflects sunlight.
- Thinking light comes out of our eyes to see things. Light goes from the object INTO the eye.
- Saying a shadow shows colours or details of the object. A shadow is only a dark outline, because light is blocked.
- Thinking a pinhole camera image is upright. Rays cross at the hole, so the image is upside down.