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School Physics Notes: Behaviour of visible light rays 1. Examples of refraction

Visible light  1. Investigating REFRACTION of visible light rays: including air-block prism interface, air-water interface, total internal reflection e.g. in fibre optic cables

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INDEX of physics optics notes on visible light rays involving reflection, refraction and diffraction


1. Investigating REFRACTION of visible light rays

See also Refraction of waves and scientific model

air-block prism interface, air-water interface, total internal reflection e.g. in fibre optic cables

Know and understand that waves can undergo a change of direction when they are refracted at an interface.

Refraction: The bending of the light ray at an interface between two media of different density.

Some sketches of light rays passing through transparent blocks or prisms (glass or Perspex).

Know that light waves are not refracted if travelling along the normal (diagram 1 below).

1. No refraction when a light ray strikes a different medium at 90o to the surface ie 'down' the normal.

The same applies to 3 and 5 for the central ray in the diagram.

However, do not assume nothing happens! There are changes in the wavelength and speed of light, but NOT the frequency of the light rays.

2. Double refraction through a rectangular glass block at the air/glass interfaces, note that when the ray emerges back into air its path is parallel to the original incident ray.

3. Refraction of light rays at the two surfaces of a diverging concave lens.

4. Refraction of light rays at two of the surfaces of a triangular glass or plastic prism.

5. Refraction of light rays at the two surfaces of a converging concave lens.

 

Two examples of light rays bending when passing from one medium to another

1. refraction image distortion of light rays by passing through glass of water gcse physics igcse 2.object in water seems bent distorted due to refraction of light rays through water air boundary gcse physics igcse 

These are two examples of refraction - light ray direction changes at a boundary between two transparent mediums of different density - in this case air/glass/water.

1. The light rays from the graph paper are refracted at the boundary interfaces by both the glass and water when entering them from the air and exiting into the air.

Note the magnification as the cylindrical glass of water is acting like a fat convex lens!

2. The pencil in the water seems to be bent because the emerging light rays from it are refracted (bent) at the interface boundary between the water and air.

The end of the pencil under water, doesn't seem to be in the right place!

A full explanation of this phenomena is given further down in this section.

 

Looking in detail at two refractions situations involving visible light

Be aware that when light rays hit a boundary between two different mediums (two materials, some of the wave energy is reflected, some transmitted - refracted and some absorbed - you should be aware of all three possibilities. (See reflection page)

I refer to them as refraction A and refraction B

The speed of light varies with the medium it is travelling through and this has important consequences for the behaviour of light when passing through a boundary between two transparent media of different densities.

Examples of the speed of light in different materials:

Vacuum (no material substance) and air (very low density) ~3.00 x 108 m/s.

Glass is ~1.97 108 m/s, Perspex ~2.01 x 108 m/s, diamond 1.24 x 108 m/s

 

Waves travel at different speeds in different materials and this can result in a change of direction as the waves pass through a boundary from one material to another.

This change in direction at the boundary between two media is called refraction.

 

Refraction A: When light waves passing through a less dense medium, hit a boundary interface (not at 90o to it), and on entering a more dense medium, the light waves 'bend towards the normal' ie refraction occurs.

Refraction of light rays A from a less dense medium to a more dense medium

This happens because on entering the more dense medium, the light waves are slowed down causing the change in wave direction at the boundary interface - ray bent towards the normal.

Diagram above, and the left of the diagram below. Refraction of light B is discussed later, but it is the opposite situation to refraction of light A.

Comparing refractions A and B

The above diagram illustrates the scientific model of the wave theory of refraction of light.

You can think of the parallel lines as representing a series of points of maximum amplitude of the light waves (rather like the crests of waves eg think waves in a ripple tank, waves on the sea or ripples in a pond on throwing a stone in).

Wave theory of refraction A (light rays-waves passing from a less dense to a more dense medium):

Refraction A happens because the wavefronts of the light rays are NOT parallel to the interface boundary so the first section of a wavefront to hit the interface is slowed down on entering the more dense medium. BUT, the other section of the wavefront is moving at the original faster speed and is skewed around producing the change in direction. So it is this decrease in speed that causes the change in direction, and, in this case, the skewing round causes the refracted ray to bend towards the normal.

You can also see that in refraction A the wavelength is decreased as well as the velocity.

The frequency does NOT change.

speed of light = frequency x wavelength,

in 'symbolic shorthand'    v = f x λ   (see wave calculations)

If the frequency (f) does not change, then the velocity (v) is directly proportional to wavelength (λ).

The bigger the change in speed the bigger the change in direction - the greater the angle of refraction.

The obvious examples in your school/college laboratory are the optics experiments you do in passing light rays passing from air into more dense transparent triangular or rectangular plastic/glass blocks or triangular prisms.

As long as the material is transparent and more dense than air you get refraction of light, as long as the incident light rays strike the interface at any angle other than at 90o (angle of the normal).

You see this effect in ripple tank experiments when you abruptly go from deeper water to shallower water the waves will change direction towards the normal.

The waves slow down in shallower water and if they hit the shallower water at an angle, refraction will occur.

The waves slow down in shallower water because of increased friction with the bottom surface of the ripple tank.

In the ripple tank the refraction of the water waves has nothing to do with density, but is caused by increased friction - increase in the 'drag' effect.

You can observe the change in speed and wavelength of water waves in a ripple tank by placing a rectangular plate in to the water at an angle to the waves and you can see these changes in wavelength and speed. BUT, by using a stroboscope you can show the frequency does not change.

For ripple tank experiments see Experiments with water waves in a ripple tank

 

Extra notes on refraction A - with refraction experiments, and real life too, you often get reflection too e.g.

Light rays passing from a less dense medium to a more dense transparent medium.

You ay 2 refracted. You do get some reflection too, ray 1. You see reflections on water and in shop windows.

Note again that when light rays hit a boundary between two mediums, some of the wave energy is reflected, some transmitted - refracted and some absorbed - you should be aware of all three possibilities.

 

Refraction B: When light waves from a more dense medium, hit a boundary interface (not at 90o to it), and on entering a less dense medium, the light waves 'bend away from the normal' i.e. refraction occurs.

This happens because on entering the less dense medium, the light waves can speed up causing the change in wave direction - light rays bent away from the normal.

The obvious examples you see in optics experiments are light rays emerging from transparent plastic blocks or triangular and rectangular glass prisms, and passing out into less dense air.

Refraction of light rays B from a more dense medium to a less dense medium

Diagram above and right of diagram below. Diagram refraction A has been previously discussed, but here refraction B is the opposite situation to refraction A.

Comparing refractions A and B

The above diagram illustrates the scientific model of the wave theory of refraction.

Wave theory of refraction B (light rays-waves passing from a more dense to a less dense medium):

Refraction B happens because the wavefronts of the light rays are NOT parallel to the interface boundary so the first section of a wavefront to hit the interface is speeded up on entering the less dense medium. BUT, the other section of the wavefront is moving at the original slower speed and is skewed around producing the change in direction. So it is this increase in speed that causes the change in direction, and, in this case, the skewing round causes the refracted ray to bend away from the normal.

You can also see that in refraction B the wavelength has increased as well as the velocity.

The frequency does NOT change.

speed of light = frequency x wavelength, in 'symbolic shorthand'    v = f x λ 

See Experiments with water waves in a ripple tank

If the frequency (f) does not change, then velocity (v) is directly proportional to wavelength (λ).

The bigger the change in speed the bigger the change in direction - the greater the angle of refraction of the light rays.

You see this effect in ripple tank experiments when you abruptly go from shallower water to deeper water the waves will change direction away from the normal.

The waves speed up in deeper water and if they hit the deeper water at an angle, refraction will occur.

The waves speed up in deeper water because of decreased friction with the bottom surface of the ripple tank.

In this example the refraction has nothing to do with density, but is refraction caused by decrease in friction - reduction of the 'drag' effect.

You can observe this in a ripple tank by placing a rectangular plate in to the water at an angle to the waves and you can see these changes in wavelength and speed. BUT, by using a stroboscope you can show the frequency does not change.

For ripple tank experiments see Introduction to Waves - Ripple Tank Experiments

 

Extra notes on refraction B - with refraction experiments, and real life too, you often get reflection too e.g.

Light rays passing from a more dense medium to a less dense transparent medium.

The concept of total internal reflection needs to be introduced here

Note: Ray 2 refracted. You do get some reflection too, ray 1.

For glass, if the internal angle of incidence is over 43o you get total internal reflection and ray 2 doesn't exist.

This particular angle when the refracted ray travels along the boundary is called the critical angle.

e.g. when the angle of incidence in a medium such as water, glass or plastic, reaches a certain critical value, the refracted ray lies along the boundary, having an angle of refraction of 90-degrees.

This angle of incidence is known as the critical angle; it is the largest angle of incidence for which refraction can still occur - even if it seems strange that some of the ray travels along the boundary!

The diagram below illustrates these points.

Situation A: The angle of incidence i1 is less than the critical angle

Most of the light ray is refracted at the media boundary.

The angle of refraction is >i1 but <90o (from more to less dense medium).

The refracted ray bends away from the normal when entering a less dense medium.

Some of the light is internally reflected - but not totally.

When the angle of incidence is less than the critical angle, little reflection takes place.

Situation B: The angle of incidence i2 equals the critical angle

Although some of the light is still internally reflected, most of the ray is refracted through an angle of 90o and travels along the boundary.

The angle of refraction is >i2 but = 90o.

When the angle of incidence is equal to the critical angle, much more reflection takes place - but still not total.

Situation C: The angle of incidence i3 is greater than the critical angle

No refraction takes place and the ray is totally internally reflected.

When the angle of incidence is greater than the critical angle, total internal reflection occurs.

This phenomenon is exploited when glass fibres (optic fibres) are used to transmit information using infrared.

Every transparent material has its own critical angle.

Glass can range from 30o to 42o, Perspex plastic 42o, diamond 24o and water 49o.

You can investigate all of this behaviour with simple ray box experiments with glass blocks.

The property of total internal reflection is used to transmit information through glass or plastic fibres using infrared and visible light beams.

The angle of incidence of light/infrared beam is always greater than the critical angle, so you always get total internal reflection and no energy is lost to weaken the signal.

 

This sort of internal reflection is part of the explanation of the formation of a rainbow.

 

The origin of the optical illusion when observing an object at an angle in water.

When you observe an object half in water and half in air e.g. poking a stick into still water, you see a 'bent' distorted image, because, the light rays from the object are bent at the air-water interface because of refraction.

If you think of the actual object at the start of the incident ray, you think the object is higher up to the right compared to where it actually is - just follow the line back from the emerging refracted ray.

You are dealing with a 'real' (deeper) and 'apparent' (shallower) depth - can be a bit disconcerting! and take care when diving into swimming pools or off rocks at the seaside - the bottom might not be quite where you think it is!

 

You can observe both refraction situations A and B when doing the ray box light experiments with a transparent rectangular block of glass or Perspex.

You do the experiment on white paper.

(i) Draw around the block with a pencil.

(ii) Direct the beam of light through the block at different angles.

(iii) For each angle mark on dots where the rays enter and leave the glass block and join them up to complete the ray diagram. Allow e.g. ~5 cm of dots on each side of the block.

the green dotted vertical lines are the two normals.

angles 1 and 3 are angles of incidence

angles 2 and 4 are angles of refraction

Remember, when a ray enters a more dense medium (air ==> glass), the ray bends towards the normal, and on entering a less dense medium (glass ==> air) the ray bends away from the normal

there maybe a little reflection of incident rays 1 and 3, but most of the rays are refracted.

 

If the waves hit the interface at an angle of 90o (perpendicular, diagram 1 above) to the interface between the two mediums, there is still a change in speed and wavelength, but there is NO change in direction, NO refraction and the wave frequency remains the same. In all the other cases 2 to 4, refraction can occur.

Wave theory to explain what happens and what doesn't happen.

A: When the waves pass from a less dense medium to a more dense medium the waves decrease in velocity at the media boundary and the wavelength also decreases.

B: When the waves pass from a more dense medium to a less dense medium the waves increase in velocity at the media boundary and the wavelength also increases.

In both cases the frequency of the light remains unchanged and in both cases no refraction takes place - no change in direction.

There is no refraction because the wavefronts of the light rays are parallel to the interface boundary so no section of a wavefront is skewed round because another section is NOT being slowed down or speeded up on entering another transparent medium of different density.

You can observe this in a ripple tank by placing a rectangular plate in to the water parallel to the waves and you can see these changes in wavelength and speed. BUT, by using a stroboscope, you can show the frequency does not change.

 

INDEX notes: Visible light rays - reflection, refraction and diffraction


Keywords, phrases and learning objectives for the behaviour of visible light rays

Investigating refraction of visible light rays: including air-block prism interface, air-water interface, total internal reflection e.g. in fibre optic cables

  • Know and understand what happens when light beams pass through prisms.

  • Know and understand that phenomena of total internal reflection and critical angle.

  • You need to understand the concept of critical angle but knowledge of the values of critical angles is not required.

  • Know and understand that visible light can be sent along optical fibres because of total internal reflection.

    • Examples of use you should know about include the endoscope for internal imaging,

  • The laser as an energy source for cutting, cauterising and burning.

    • You should know about its application and use in eye surgery,

    • but knowledge of how lasers work is not required.

Check out your practical work you did or teacher demonstrations you observed, all of this is part of good revision for your module examination context questions and helps with 'how science works'.

experiments - investigation of refraction using a ray box

carrying out refraction investigations using a glass block or triangular prism


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INDEX notes: Visible light rays - reflection, refraction and diffraction

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