Science labs › Physics › Bending Light
Shine a ray into a glass block. How much does it bend?
Bending Light lab · Last updated: 16 October 2026
A thin ray of light shines into a glass block. You change the angle and watch the ray bend as it enters the glass and bend back as it leaves. Why does light change direction at all?
What you'll discover
- Light bends when it passes from air into glass, because it slows down.
- It bends towards the normal going into glass and away from it coming out.
- A ray that hits the glass straight on goes through without bending.
The science in plain words
Light travels at about 300,000 kilometres per second in empty space and almost as fast in air. In glass it slows to about two thirds of that speed. When a ray enters glass at a slant, one side of the beam reaches the glass first and slows down first, so the beam swings round, just as a toy car turns when one wheel runs onto carpet. This bending is called refraction.
Going from air into glass, the ray bends towards the normal, the imaginary line at right angles to the surface. Coming out the other side, it speeds up again and bends away from the normal. For a block with parallel sides, the ray leaves travelling in its original direction, just shifted sideways. A ray that hits the surface straight on slows down but does not bend.
How strongly a material bends light is measured by its refractive index. Glass is around 1.5 and water about 1.33.
Refraction explains why a straw looks bent in a glass of water and why a pool looks shallower than it really is. It is also how lenses in spectacles, cameras and our own eyes focus light.
Where it fits
- India, NCERT: Class 10 science, Chapter 9: Light: Reflection and Refraction
- United States: Grades 6 to 12
- Next Generation Science Standards: MS-PS4-2
Common questions
Why does light bend in water or glass?
Light travels more slowly in water and glass than in air. When it enters at a slant, one side of the beam slows first, so the beam changes direction. This is called refraction.
Why does a straw look bent in water?
Light from the underwater part of the straw bends as it leaves the water. Your brain assumes light travels in straight lines, so it sees the straw in the wrong place.
What is refractive index?
It is a number that tells you how much a material slows light and bends it. It is the speed of light in a vacuum divided by its speed in the material, about 1.5 for glass.
Related labs
- Candle, Lens and Screen: Slide the screen until the candle is sharp. Why is its picture upside down?
- Prism Rainbow: Split white light into a rainbow with a glass prism.
- Glowing Glass Rod: Shine a laser into a bent glass rod. Does the light follow the bend?
See all physics labs or every Taz science lab.
Try it yourself
The Bending Light lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.