KS4 Waves : Refraction

By the end of this lesson you should be able to: Define refraction Draw ray diagrams showing refraction of light by a glass or Perspex block List and explain everyday examples of refraction

Animation showing refraction at the air/glass boundary

Investigating refraction Apparatus: Power pack Ray box Slit Perspex block Protractor The ray box will get very hot, be careful when handling it.

Exam tip You will lose marks in an examination when you are drawing a light ray if you do not: 1. Use a ruler. 2. Add an arrow to show direction.

What to do…… Draw around the Perspex block on a piece of paper.

The normal Mark on an imaginary line known as the NORMAL, it is perpendicular to the surface of the block. For the light ray entering the block and the light ray leaving the block mark each ray with two crosses. Repeat several times with the light ray entering the block at different angles. Draw in the incident ray, emergent ray, remove the block and then join up the two rays.

What happened…… As the light ray moved from air into Perspex? As the light ray moved from Perspex into air? If the angle of incidence = 0°? What do you notice about the incident ray and the emergent ray?

Results Does the angle of incidence…… If the light ray entered the block parallel to the normal then it travels through undeviated. …affect the angle of refraction? If the incident ray enters the block at an angle to the normal then the direction of the ray changes as it enters and leaves the block, the light ray has been refracted.

Measure the angles of incidence and refraction and fill in the table below Angle of incidence ( i) Angle of refraction ( r)

Air to Perspex angle of incidence > angle of refraction i > r As the light ray moved from air into Perspex it moved towards the normal. If light rays move from a less dense medium (air) to a more dense medium (Perspex) they ‘bend’ towards the normal. i > r

Perspex to Air angle of incidence < angle of refraction i < r As the light ray moved from Perspex into Air it moved away from the normal. If light rays move from a more dense medium (Perspex) to a less dense medium (air) they ‘bend’ away from the normal. i < r

Angle of incidence = 0° Un-deviated light ray When the angle of incidence is 0 the light ray is not deviated from its path. Un-deviated light ray

Animation to show what happens to a ray of light passing through a rectangular block of glass

Revision tip Remember the word: TAGAGA Towards (normal) Air Glass Away (from normal)

Fast and slow In the water If you were running along a beach and then ran into the water when would you be moving slower, in the water or on the beach? In a similar way as light moves from one medium to another of different density the speed of light changes. Do you think light moves faster or slower as the density of the medium it travels through increases? In the water Light moves faster through less dense media.

The speed of light Light travels at 300 000 km/s in a vacuum, as it enters denser media the speed of light decreases. Perspex must be denser because light travels slower through Perspex than water. Looking at the chart, which do you think is denser, Perspex or water?

Why does light change direction as it enters a material? Imagine a car driving from the road into a muddy field. In the muddy field it slows down as there is more friction. If it enters the field at an angle of incidence of other than 0 then the front tyres hit the mud at different times. Tyre one hits the mud first and will move more slowly than tyre two.This causes the car to turn towards the normal. When the car leaves the mud for the road, tyre one hits the road before tyre two and this causes the car to turn away from the normal. Tyre 1 Tyre2 Road Mud

No change in direction If the car approached the muddy field at an angle of incidence of 0° then both front tyres would hit the mud at the same time. The tyres would have the same speed relative to each other so the direction of the car would not change, it would just slow down.

Same for light When light hits a medium at an angle to the n_____ the light ‘bends’ in a similar way to that described for the car in a muddy field. Part of the light ray s____ d___ before the rest and this causes the change of d______ (for light moving from a l___ dense medium to a m___ dense medium). If the light enters a new medium along the normal (i = 0) then it does not ‘bend’ because all of the light ray slows down at the s___ r___. ormal ame ate lows own irection ess ore

Refraction : Effects of Refraction  Many visual effects are caused by refraction.  This ruler appears bent because our brain assumes the light coming from the ruler hasn’t been refracted.  Would the ruler appear more or less bent if the water was replaced with glass? More bent, because glass is more dense than water.

Refraction : Magic coins  Place a coin in the bottom of a bowl and clamp an empty cardboard tube so that it points above the coin. Gradually add water to the bowl and watch the coin through the tube float up - can you explain this?

Refraction : Apparent Depth  The rays of light from the coin get bent [refracted] as they leave the water.  Your eye assumes they have travelled in straight lines.  Your brain forms an image at the place where it thinks the rays have come from - the coin appears to be higher than it really is.

The Archer fish The Archer fish is a predator that shoots jets of water at insects near the surface of the water, say on a leaf. The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey. Image of prey Prey location The fish does not aim at the refracted image it sees but at a location where it knows the prey to be.

Animals and human hunters Animals (including humans) allow for refraction when hunting fish in water. The animals do not aim at the fish (it is just the refracted image), instead they aim at a location where they know from experience the fish actually is. Image Actual location

Refractive index We can study refraction of light by comparing its speed in air to that in a medium. A number called the refractive index is the ratio of these two speeds: Refractive index = speed of light in air speed of light in substance Example: The speed of light in air is 300 000 000 m/s, the speed of light in water is 225 000 000 m/s. What is the refractive index of water? 1.33

Calculating refractive index Material Speed of light in material Refractive index Air 300 000 000 Water 225 000 000 Diamond 120 000 000 Perspex 200 000 000 1.0 1.33 2.4 1.5

Using Refraction : Lenses Summary  There are two main types of lens: Convex Concave  Convex lenses work by bending [refracting] rays of light to a principal focus.  The distance from the centre of the lens to the principal focus [F] is called the focal length [ƒ].  The image formed by a convex lens is inverted [back-to-front and upside-down].  The thicker the lens, the shorter the focal length[ƒ].

Using Refraction : Lenses  The lens refracts all the rays to a point called the principal focus [F].  The distance between the centre of the lens and F is called the focal length [].  Imagine parallel rays of light from a distant object hitting the lens.  Draw normal lines where the rays enter the air [at 90º to the surface].  Use the first refraction rule to work out the ray direction.  Work out the direction of the refracted rays using the second refraction rule. F  Draw normal lines [at 90° to the surface] for each ray.  A lens can be thought of as a series of prisms.  When light enters a less *dense medium [e.g. air], it bends away from the normal.  When light enters a more *dense medium [e.g. glass], it bends towards the normal. ƒ

What do you think happens when… They pass through the focal point of the lens. Parallel light rays strike a convex lens? Form a parallel beam if they pass though the focal point (F). Diverging light rays? F

ƒ Using Refraction : Lenses - Finding  Hold the lens in the other hand and move it closer to the screen until a clear image appears. Hold a plain white screen in one hand. Chose a distant object [to get parallel rays of light]. Use a ruler to measure the distance between the lens and the screen - this is the focal length [ƒ]. ƒ

Refraction : Lenses 1. Find the focal length [ƒ] of your lens. 2F F F 2. Fix the lens to the centre of a metre rule and mark the distances F and 2F either side of the lens. 3. Place the candle >2F away from the lens and move the screen until an image appears and record observations. 4. Repeat for the candle at 2F, between 2F and F, at F and between F and the lens.

Magnified or diminished Results Object position Image Position Real or virtual Magnified or diminished Inverted or erect >2F at 2F between 2F and F at F between F and lens

Refraction : Lenses O I  Object >2F away 2F F  The image [ l ] is formed between F and 2F away from the lens, is inverted and diminished.

Refraction : Lenses O I  Object at 2F 2F F  The image [ l ] is formed at 2F away from the lens, is inverted and the same size.

Refraction : Lenses O I  Object between 2Fand F away 2F F  The image [ l ] is formed further than 2F away from the lens, is inverted and magnified. I

Refraction : Lenses O  Object at F away 2F F  The image [ l ] is formed at infinity - the rays never meet [we use this set-up for searchlights].

Refraction : Lenses I O  Object between F and lens 2F F  The VIRTUAL image [ l ] is formed on the same side of the lens as the object, is the right way up and magnified.

Magnified or diminished Results Object position Image Position Real or virtual Magnified or diminished Inverted or erect >2F at 2F between 2F and F at F between F and lens between F and 2F real diminished inverted at 2F real same size inverted real magnified inverted > 2F at infinity same side as object virtual magnified erect

Refraction : Lenses Magnification = Distance from lens to image Distance from object to lens

Which of the following is the most dense? Air Water Glass Lead 

When light changes direction as it moves from one medium to another we call this effect what? Reflection Refraction Diffraction Total internal reflection 

 It continues with no change of direction It bends towards the normal If a ray of light moves from a more dense medium to a less dense medium at an angle to the normal what happens? It continues with no change of direction It bends towards the normal It bends away from the normal It stops 

What happens to the speed of light as it moves from air into glass?  Decreases Increases No effect Decreases and increases

If a ray of light moves from air to glass parallel to the normal what happens? No change in direction It bends away from the normal It bends towards the normal It stops 

If light travelling through a medium has a speed of 150 000 000 m/s If light travelling through a medium has a speed of 150 000 000 m/s. What is the refractive index of the medium? 2.6 0.5 2.0 1.5 

Can you…… Explain what refraction is? Describe what happens to a light ray if it enters a medium of different density at an angle? Describe what happens to a light ray if it enters a medium of different density along the normal? Describe examples of refraction? Draw ray diagrams depicting the refraction of light? Calculate the refractive index for a medium?