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How Light Travels to Let You See · We can see objects only when light travels from a source, bounces off the object, and enters our eyes. · 4-PS4-2

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How Light Travels to Let You See

with Lumi

Lumi the glowing firefly guide hovers beside a cozy lamp in a bedroom, tracing a dotted golden path with one wing from the lamp to a shiny red apple on a table, then onward to a child's wide-open eye.

Explain that we see an object only when light reaches our eyes from it.
Trace the three-step path of light: source, object, then eye.
Identify everyday objects as light sources or non-sources.
Predict that an object cannot be seen in total darkness because no light bounces off it.

Key terms

light source: something that makes its own light
reflect: to bounce light off a surface
eye: the body part that takes in light
darkness: a place with no light at all

Light Starts and Bounces

Seeing always needs light, and light takes a three-step trip. First, light starts at a source, like the Sun, a lamp, or a glowing tail. Things like apples and chairs do not make their own light. Next, the light zooms in straight lines until it hits an object, then it bounces off, just like a ball off a wall. Different objects bounce light in different ways, which is why each one looks a little different to you.

Light Reaches Your Eye

The last step is when some of that bounced light travels straight into your eye. Your brain then figures out the shape and color you are looking at. Your eyes do not shoot out beams; they only take in light that comes to them. If no light reaches your eye, there is nothing to see. That is why even a bright red apple is invisible in a room that is truly dark, with no light to bounce off it.

Worked examples

How does light travel so you can see a toy on a table?

Light starts at a source, like a lamp or the Sun.
The light bounces off the toy in straight lines.
Some of that bounced light goes into your eye.

Answer: Light goes source, then toy, then eye, so you can see the toy.

Can you see a red apple in a room with no light at all?

An apple makes no light of its own.
It can only reflect light that hits it.
With no light to bounce, nothing reaches your eye.

Answer: No, you cannot see the apple because no light bounces to your eye.

Hi explorer, I'm Lumi! Have you ever wondered HOW your eyes see a toy across the room? Here's the secret: seeing always needs light, and light has to make a three-step journey. Step 1 — STARTS at a SOURCE. A source is something that makes its own light, like the Sun, a lamp, or my glowing tail. Things like an apple or a chair do NOT make their own light — they can only bounce light that hits them. Step 2 — BOUNCES off the object. Light zooms in straight lines until it hits something solid, then it bounces away — kind of like a ball off a wall. Different surfaces bounce light in different directions, which is why every object looks a little different. Step 3 — ENTERS your eye. Some of that bounced light travels straight into your eye. Your brain (the thinking part inside your head) then figures out what shape and color you are looking at. No light reaching your eye means nothing to see — even a bright red apple is invisible in a truly dark room. If you feel stuck during the activity, look at Lumi's dotted golden path in the picture — follow it arrow by arrow from left to right and it will show you the correct order.

Activity

Sort each item into the correct group: light SOURCE (makes its own light) or REFLECTOR (only bounces light from somewhere else).

Practice

Name two things in your room that make their own light.

Explain why you cannot see your toys in a totally dark room.

Common mistakes to avoid

eyes shoot out beams to seeEyes only take in light; they never send beams out to objects.
you can see in total darknessWith no light to bounce into your eye, you cannot see anything.

Check your understanding

What must happen before your eyes can see a toy sitting on a table?

You are in a room with no light at all. Can you see a red apple on the floor?

Which of these is a light SOURCE — something that makes its own light?

Recap

We see things only when light reaches our eyes. Light starts at a source, bounces off an object, and then travels into your eye, where your brain makes the picture. In total darkness, no light bounces, so you cannot see anything at all.

Reflect

Which light source helped you see things in your home today?