Refraction
Light, Sound and Waves

Refraction of light

for 14-16

Refraction can produce familiar tricks and images involving distortions. It also provides a simple way of measuring refractive index.

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Refraction in a tank of water

Refraction
Light, Sound and Waves

Refraction in a tank of water

Practical Activity for 14-16

Demonstration

Refraction at an air-water interface and total internal reflection.

Apparatus and Materials

  • Plastic tank, rectangular
  • Lamp, stand, housing and holder
  • Multiple or triple slit
  • Holder for slits
  • Power supply for lamp
  • Paper, white

Health & Safety and Technical Notes

Read our standard health & safety guidance


The plastic tank needs to be approximately 75 mm deep, straight sided and about 20 x 12 cm. A transparent plastic lunch box is ideal.

The lamp does survive being immersed in water ( not the electrical connections), but an additional precaution is to put the cold lamp into the water before switching it on.

You will need to paint the inside lower surface of the tank white. The ray streaks in water will not show on white paper placed under the tank because of total reflection at the air surface between tank and benchtop.

Procedure

  1. Fill the rectangular plastic tank with clean water, so that the water level is below that of the electrical connections to the lamp. Direct rays from the lamp at the sides of the tank. The streaks will show on white paper on the table while in air. In water they will not show, unless the inside surface of the base of the box is painted with flat white paint.
  2. With the lamp outside; send rays towards the box. The rays will bend when they strike the water surface at various angles.
  3. Carefully place the lamp inside the tank, with the lamp, but not the electrical connections, under water. With the multiple slit inserted, the box will send ray streaks through the water. Observe what happens when those streaks meet the water-air surface. Refraction occurs as they emerge into air, and it should be possible to see total internal reflection.

Teaching Notes

  • It is possible to measure the angles of incidence i and refraction r (emergence) and check them using Snell's Law (n 1 sin i =n2 sin r ).
  • Light travelling from a more to a less optically dense medium is bent away from the normal (a line drawn perpendicular to the surface at the point where the ray emerges). When the lamp is put into water, the effect of only one interface is seen. Otherwise, it becomes very complicated, especially when total internal reflection occurs.
  • Total internal reflection can often be seen when looking through a window; an image of the surroundings behind you gets reflected.

This experiment was safety-tested in January 2007

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Further refraction demonstrations

Refraction
Light, Sound and Waves

Further refraction demonstrations

Practical Activity for 14-16

Demonstration

Three simple demonstrations of the refraction of light at an air-water interface.

Apparatus and Materials

  • For coin in beaker...
  • Wide-topped glass container, e.g. 400 cm3 beaker or jam-jar
  • Coin
  • For the bent stick...
  • Glass container
  • Stick or pencil

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Coin in beaker
  2. Put a small coin, for example a five pence piece, at the far side of the container and arrange cardboard screens to keep the coin out of sight. Pour water carefully into the beaker without moving the coin. The coin's image will move into sight.
  3. The bent stick
  4. Put a straight stick or pencil into a tank of water or sink at an angle of about 45°C and look at it from one side, and from above.
  5. Apparent depth of a pond
  6. A large sink or swimming bath appears shallower than it really is when it is filled with water. The farther parts of the bottom also appear to curve up towards the observer. That this is an illusion can be checked by looking from the other side. It is best to look across a swimming bath as the actual depth usually varies from end to end. It helps to get one's eyes near the surface as illustrated. This can also be done in a bath. If the water just covers one's toes, one's feet seem to lengthen greatly as they are raised out of the water.

Teaching Notes

  • These experiments might be done as quick demonstrations by the teacher, or students can be encouraged to do them for themselves at home.
  • If rays are drawn to show how light reaches the eye, the refraction that occurs at the air/water interface in each of the demonstrations can be seen.

This experiment was safety-tested in January 2007

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Brandy tears

Refraction
Light, Sound and Waves

Brandy tears

Practical Activity for 14-16

Demonstration

This can be shown to a sixth form class as a fun demonstration of light interference.

Apparatus and Materials

  • Laser or different colour lasers if available
  • Small rectangular glass box as used for optics expts. (e.g. use a 6 cm hollow cube)
  • Small right-angled 45 degree glass prism to fit inside the box with the right angle in one corner
  • Bottle of (cheap) brandy - works better than ethanol
  • Screen

Health & Safety and Technical Notes

The laser should be positioned so the beam cannot fall onto the eyes either directly or indirectly.

Be careful of laser reflections from the face of the glass box.

Don't drink the brandy!!

Read our standard health & safety guidance


The brandy can be re-used although eventually the alcohol content becomes too low.

It is possible to use a brandy glass and brandy rather than the apparatus used here but it is quite tricky to get working well.

The glass box and prism must be clean and dry.

Procedure

  1. Arrange the box and prism on the bench with the laser set to shine in through the side and into the prism, passing perpendicularly through the prism face.
  2. The light will internally reflect from the back face and emerge from the third side of the prism; then pass onto the screen, which should be set around two metres from the glass box.
  3. Pour some brandy into the box until the level is about 1 cm below the point on the prism face where the laser beam is entering.
  4. After a while brandy tears will creep up the prism face.
  5. Adjust the laser so that it hits one of the tears (be patient).
  6. A striking interference pattern will be seen on the screen and will be continually changing. It is best seen in a darkened room.
  7. If lasers of different wavelengths are available, the different fringe separations are obvious.

Teaching Notes

  • Without the brandy the laser totally internally reflects from the prism's back face.
  • Brandy in contact with the face changes the critical angle at that point (because the change in the speed of light between glass and brandy is less than the speed change between glass and air) with the result that the light now enters the brandy tear, reflects from the back surface (a complex shape) and continues back through the prism to the screen, interfering with itself and producing the fringes on the screen.
  • Alcohol continually evaporates from the brandy tear, changing its concentration and so the surface tension, and hence the angle of contact with the glass. As a result, the liquid in the tear is flowing up and down the glass all the time, changing the shape and thickness of the tear and so causing the interference patterns to shift in a fascinating way. The effect is rather beautiful and very soothing.

This experiment was submitted by Rod Smith from Cranbrook School in Kent.

This experiment was safety-tested in February 2008

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Teaching ray optics

Reflection
Light Sound and Waves

Teaching ray optics

Teaching Guidance for 14-16

At introductory level, simple experiments can help students to realize that light travels in straight lines and that an object is seen when light from the object enters the eye. A lens bends light rays so that the rays pass through an image point and we think we see the object at that point.

Treated as open-ended experiments they show students the way in which light behaves with real lenses in optical instruments.

Photograph courtesty of Jim Jardine

Most of the experiments described on this website are suitable for intermediate level courses. After completing them, students should be able to draw a diagram of light rays (not formal ray construction diagrams) showing the following.

  • Rays travel out from an object point in all directions, going fainter as they go farther.
  • All rays from a remote object point pass through an image point.
  • Rays from a remote object point which pass through a lens and proceed to a real image point after the lens, continue straight on through that point.
  • Rays from an object point which pass through a lens forming a virtual image emerge along lines that appear to come straight from the image point.
  • Every ray aimed at a central point in a lens (called the optical centre) passes through undeviated.

The real behaviour of rays falls short of the ideal of passing through images exactly. Students will see this and learn a little about correcting for that aberration.

The ray optics equipment suggested in these experiments looks simple, but some practical skill is needed to get the best out of it. Teaching notes provided with each experiment will help you ask the right questions of students struggling to get results.

You will be better prepared for student questions if you try out the experiments carefully beforehand. It is also advisable to read traditional textbooks that go beyond what students need to know for examination purposes. For example, knowing that the minimum distance between object and image is four times the focal length of a converging lens will enable a teacher to choose a lens that suits the length of a demonstration bench.

A well-organized cafeteria of equipment, under teacher control, will encourage students to do their own experimenting. In this way, extension work for faster students can be encouraged.

At intermediate and advanced level, ripple tanks can be brought in when needed, to show reflection or refraction for example. Wave theory predicts that all parts of a wavefront starting from a small light source arrive in phase at the image. This requires all paths from the object to take the same time.

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