Class 12 Physics (India)
- Refraction and Snell's law
- Refraction in water
- Refraction through glass slab
- Refraction and light bending
- Snell's law example 1
- Snell's law example 2
- Absolute & relative refractive index
- Relative & absolute R.I. connection
- Reflection and refraction questions
- Refractive index and the speed of light
- Connection between relative and absolute refractive indices
- Total internal reflection
- Worked example: Total internal reflection
- Total internal reflection
- Applications of Total Internal Reflection
You might have heard people talk about Einstein’s speed of light, and that it’s always the same. The part that most people leave out is that this is only true in a vacuum—when there’s no pesky molecules of air or water to slow it down. But when light moves through a more familiar medium like air, it moves more slowly due to the interactions of individual photons with the molecules in the material. In general, the more optically dense the medium, the slower the light will move. So what happens to the light when it goes from one medium to another?
Light changes speed
Imagine that you and your friends are at the beach. You all decide to swim together, so you link arms and approach the water as a straight line. As you start walking into the water, you all slow down, because it’s harder to walk through water than through air. So imagine that you approach the water at an angle relative to the shoreline. The person on the end will meet the water and slow down first, then the next person in line, then the next, until everyone is walking through the water. Because one end of the line slowed down before the other end, the line of people becomes crooked, with the people who are still on the beach at a different angle relative to the water’s edge than the people who have already entered the ocean.
Beach example diagram
You can say that the water bends the path of of each individual toward a normal line drawn perpendicularly to the shoreline, since the people still on the shore are bent further away from the shoreline than those in the water.
The same thing happens to a light ray when it moves from air to water, or from any fast medium to a slow medium: it bends toward the normal.
Light ray bending to normal diagram
Least time principle
Another way to think of this is to imagine you and your friend are racing out to a raft in the middle of a lake. You have to travel across a beach and then through the water.
Beach and raft example diagram
Your friend decides to make a beeline for the raft and runs down the beach. You know that you can run faster than you can swim, so you head toward the water at an angle so that you run for longer than you will swim.
Beach and raft example diagram with paths overlay
Sure enough, your strategy pays off, and you get to the raft first. Your friend took the shortest path in terms of distance, but you took into account the difference in speed in each medium, and took the shortest path in terms of time. That means you took the path of least time.
Light does exactly that when moving between mediums. It takes the path that will take the shortest amount of time, when you account for the difference in speed between the mediums.
For example, imagine you are looking out the window. You have air, glass, and then air again. Glass is denser than air, so the light from outside passes from a fast medium, through a slow medium and into a fast medium again. The light will take the path from outside to your eye that spends the least time
Light diffraction through glass diagram
You can also see that the rule from earlier still applies: when the light enters the glass the ray is bent towards the normal. When it leaves the glass, it is bent away from the normal, and regains the same angle as before it entered the glass.
Light diffraction through glass diagram with overlay
Consider the following
Say you’re at the aquarium, and there’s a tank that’s totally full of water, so there’s glass over the top and sides. If you stand back a little, you can see the same stingray through the top of the tank and the wall of the tank.
Aquarium refraction diagram
Because you see the light coming through the glass at two different points, you see two images of the same stingray.
Want to join the conversation?
- i understood that light takes a short path (refracts) as it goes from rarer to denser due to more density (more obstruction from particles of medium). but who is giving it energy to move with the same speed after second refraction(from denser to rarer).(2 votes)
- Nothing gives it energy as well as nothing is taking energy from it. Light is massless, it travels at the speed of light. The speed of light however changes depending on how dense something is due to the light bouncing off of the molecules in the medium more often in a more dense medium.
Thus we shouldn't really think of the light as going "slower" in denser medium but instead as taking a longer path to get through it so it takes longer. The apparent speed over a distance (average speed) is slower but its actual speed has not changed and never does.
Thus no speed is ever lost and no energy is lost, neither is it given to the light. It's always the same.(29 votes)
- can you explain please why any pencil appears to bend in water(2 votes)
- Because the light can't travel as quickly in the water as it does in the air, the light bends around the pencil, causing it to look bent in the water. Basically, the light refraction gives the pencil a slight magnifying effect, which makes the angle appear bigger than it actually is, causing the pencil to look crooked.(6 votes)
- what if the refracted ray bends over 90 degrees?(2 votes)
- I don't understand the last stingray bit... Somebody please explain...
- This part talks about the location of the viewer who's watching the stingray. Light travels in masses, in a 'beam' rather than a ray. This tells us the refraction takes place from any point of the viewer. However, the meaasurements of the angles change.(1 vote)
- Okay, but I really didn't understand the stingray example at the end of the article. How we will be able to see two stingrays from the same position. Shouldn't we be able to see only one?(3 votes)
- How dose light know how to bend through the shortest path in the slow median?(3 votes)
- Everything in the universe wants to have minimum energy.
The longer the path that light takes in a denser medium, the more energy is to be applied by it in order to cross the medium, thus exerting more energy which is undesirable.(0 votes)
- i understood that light goes slower in denser medium. but what if i took air and a very dense medium lets say graphite. Will light pass through it and what will be it effects on light's speed and bending?(1 vote)
- how do you calculate the new speed that the light is traveling after entering the more dense medium?(1 vote)
- sin i/sin r = v1/v2 where i is the angle of incidence, r is the angle of refraction, v1 is the speed of light in air(1 vote)
- why light bends(1 vote)