Light Sources and Shadows

Core Idea

Light is a form of energy that travels in straight lines from its source. Sources of light include the Sun, light bulbs, candles, and screens. When light hits an object that it cannot pass through, a shadow forms on the other side. The size and shape of a shadow depend on the position and distance of the light source and the object. Without light sources, we would not be able to see anything.

How It's Best Learned

Use a flashlight in a darkened room to explore how shadows form behind objects. Move the flashlight closer and farther to see how shadow size changes. Trace shadows of objects on paper at different times of day using sunlight.

Common Misconceptions

• We can see objects because our eyes send out light. (We see because light bounces off objects and enters our eyes.)
• Shadows are dark because they are made of something. (Shadows are simply areas where light is blocked — they are the absence of light, not a substance.)
• The Sun is the only natural light source. (Stars, lightning, fireflies, and some deep-sea creatures also produce light.)
• Shadows are always the same size as the object. (Shadow size changes based on the position and distance of the light source.)

Explainer

Look around the room and think about what lets you see everything — it is light. Light is a form of energy that travels from a source to your eyes. The Sun is the biggest and most important light source for life on Earth, but light also comes from light bulbs, candles, fires, TV screens, and even tiny creatures like fireflies. Anything that produces its own light is called a light source.

One of the most important things to know about light is that it travels in straight lines. It does not curve around corners or bend around objects on its own. You can see this when a beam of sunlight comes through a window — it makes a straight, bright stripe on the floor. Because light travels in straight lines, when it hits something it cannot pass through, a dark area called a shadow forms on the opposite side.

Shadows change depending on where the light is. When the Sun is low in the sky, like early in the morning or late in the afternoon, shadows are long and stretched out. At noon, when the Sun is high overhead, shadows are short. If you move a flashlight closer to an object, the shadow on the wall grows bigger. Move the flashlight farther away, and the shadow shrinks. This is because the angle and spread of the light rays change with the position of the source.

We see objects because light bounces off them and into our eyes. When light hits a red ball, the ball absorbs most colors of light but bounces red light back. That bounced red light enters your eyes, and your brain says "red ball." In a pitch-dark room, you cannot see the ball at all because there is no light to bounce off it and reach your eyes. Light sources, straight-line travel, and shadows are the building blocks for understanding everything about how we see the world.