- Define refraction.
- What happens to light when it enters a more optically dense transparent medium at normal? at an angle?
- What happens to light when it enters a less optically dense transparent medium at normal? at an angle?
- Under what conditions does light NOT bend when moving from one transparent medium to another? Hint: There are two ways this can happen.
- Optional challenge question (involves math!): Sunlight traveling through space takes 8 minutes & 20 seconds to travel to earth. If space was a diamond, how long would it take for sunlight to travel to earth? Show your math! (We'll do this one in class!)
- Draw and label a ray diagram (on the worksheet that is provided) that illustrates what happens when a single beam of light enters a transparent glass rectangular block at an angle. Don’t forget to indicate where normal is with a dashed line and use a ruler so you can draw straight lines.
Refraction -the bending of light
(E. Refraction – the bending of light due to changing speed as light passes through (or, transmits from) one transparent material into another (of different optical densities) at an angle (other than normal)
1. In a vacuum such as space, light travels 300,000 km per second (or 186,000 miles/second)
a. Called ‘c’ (Refractive Index, R.I. = 1.0)
2. As light is transmitted through more optically dense substances, it slows down. It will bend if it enters at an angle other than normal (90 degrees)
a. Optical density is not the same as mass density (Ex. oil is less dense by mass than water - remember that oil floats on water-, but it is more optically dense, so light slows down more in oil than in water)*
i. In air, light travels only slightly less than c (R.I. = 1.00029)
ii. In water, light travels at 75% c i.(R.I. = 1.33)
iii. In oil, light travels at 66% c i.(R.I. = 1.47)
iv. In a diamond, light travels at 41% c (R.I. = 2.419)
*Note that it often is the case that something that has greater mass density is also more optically dense, but there are exceptions, which is why we distinguish mass and optical density. Another way of talking about optical density is the "refractive index", which is a measure of optical density of a material compared to a vacuum. Nothing travels faster than light in a vacuum. So transparent materials with a higher refractive index (R.I.) slow light down more, causing it to bend more.
*Note also that the two transparent media need to be at different optical densities for the light to bend
3. When light slows down, it bends towards normal and, when it speeds up, it bends away from normal**
**Remember when we went outside to march into the 'slower' grass? At the bottom of the page is a video that demonstrates this.
4. Light refraction causes images to appear bent, broken and/or larger because the light bends through the medium (like water) before it reflects back to your eye.
Comparing light to running into water at a beach, or a wheel into a ditch
Imagine that you are running from the beach into water. When your legs hit the water, they slow down, but the rest of your body keeps going. This causes you to face-plant into the water (bend toward the water) because half your body is slowing down when the other half is still going fast.
Imagine that when you are driving down the road. If your right wheel goes into the muddy side of the road, it will slow down. The left wheel will continue at the original speed. This causes the car to swerve to the right into the ditch. Not a good idea!!
The same thing is happening when light enters another transparent medium at an angle. The part that enters the more optically dense medium slows down first, causing the faster light to bend towards the more dense material.
The opposite happens when you are going from a more dense medium into a less dense medium. In this case the light that emerges first will speed up and bend away from the medium.
The videos and interactives below should help with your understanding of the refraction of light
"Marching out" Refraction
The video on the left is a more advanced lesson on the topic of refraction. This clip shows what we did when we marched outside to demonstrate refraction. For the full, unedited version that talks about Snell's law (you DO NOT have to know this!), go to this website.