# 03Reflection and refraction

Li03TA of the Light topic
• ## 01 Reflection of light in plane mirrorsLi03TAnugget01 Activity

### Rolling the ball

What the activity is for

The following activities can be used to introduce the law of reflection of light.

What to prepare

• small ball
• projector and large plane mirror
• narrow beam sources, plane mirrors and protractors
• an interactive diagram, and the means to display it to the class

What happens during this activity

A short opening activity does not involve light but instead uses a small ball. Gather the class round one side of the room and take the ball. Roll it along the floor so that it hits the wall at about 45 degrees and bounces (reflects) away. Repeat this at a number of different angles, without comment, then ask the class what they notice about how the ball bounces off the wall.

Most pupils have an intuitive sense that the ball bounces away from the wall in a symmetrical way and that the angles are equal. If you open up the discussion here, you will soon find yourself deep in matters of snooker, basketball and the like.

Make the point that the aim of the lesson is not to think about balls bouncing off walls, but of light being reflected from mirrors.

Teacher: Where will the beam of light end up?

Set up a projector so that it casts a beam of light across the lab. You might place the projector so that it is somewhere in the body of the room, with the beam being directed towards the front. Switch all of the lights out and locate the beam with a white screen (a large piece of paper will do).

Now take a reasonably large plane mirror and hold it up towards the light beam. Before you place the mirror in the beam, ask the pupils to predict what they think will happen to the beam of light when it strikes the mirror.

You are likely to be told that the beam will be reflected. Ask the class to predict in which direction the light will be reflected. Who will it hit in the class? Will it be similar to the ball bouncing off the wall?

Now place the mirror in the beam and see where it lands. Repeat this, holding the mirror at different angles to the incident beam. Take care not to shine the beam into the face of any pupil.

It will soon become evident that the light is reflected from the mirror in the same way that the ball is reflected from the ball.

The angle going in is equal to the angle coming out.

In this final activity, checking out the law of reflection of light, direct the pupils to work in pairs with narrow beams and plane mirrors to check the emerging idea that when light is reflected the angle going in is equal to the angle coming out.

In practice the biggest problem here is created by the need to measure angles up to the normal line.

• ## 02 Your very own mirror invention!Li03TAnugget02 Activity

### Inventing using plane mirrors

What the activity is for

This activity provides the opportunity for pupils to apply their knowledge of the law of reflection of light from a plane mirror in an imaginative way.

What to prepare

• devise a starter support sheet that will get your class off on the right lines, if necessary

What happens during this activity

Tell the class that they are going to use their knowledge of the law of reflection of light to invent a device, based on flat mirrors, which no-one else has thought of before. Encourage the class to let their imaginations fly. The device might, for example, be:

• Used to look at boils on the back of your neck.
• Attached to the end of your shoe so that you can see under tables.
• Used to see over high walls around football grounds.
• Set up so that you can watch the TV in the living room from the stairs, without being detected by your parents.

Directions:

Think up an unusual invention that works using flat mirrors. Don't make it too complicated!

Make a careful drawing of your invention, showing each of the mirrors clearly. Draw in a single ray to show how it passes around the mirrors and to your eye.

At each of the mirrors be sure to include the normal line and mark the angles of incidence and reflection.

Although the device itself may not be altogether serious, you should insist that the diagrams are accurately drawn to demonstrate the law of reflection of light.

• ## 03 Survey: lighting at homeLi03TAnugget03 Activity

### Lighting at home

What the activity is for

This activity helps to make the link between studies of light at school and lighting in the home. It will provides lots of points of discussion relating to the pros and cons of different kinds of bulbs.

What to prepare

• devise a tabulated starter support sheet that will get your class off on the right lines, if necessary

• room
• light function
• light fitting
• bulb
• colour
• power
• direct/indirect lighting

What happens during this activity

This homework activity involves the pupils in making a survey of the lighting in their home. You will need to provide the class with a table in which they can record their findings.

Clarify with the class what each of the headings means – their descriptions should be simple enough for the rest of the class to understand.

Room: what is the room used for?

Light fitting: the kind of fitting, e.g. chandelier, reading lamp, strip light

Type of bulb: filament, fluorescent, energy-saver, halogen

Colour: most will be white

Power (in watts): If electrical power has been covered in lessons on electricity this exercise will provide a useful link. Otherwise introduce the idea that the power of the bulb is a measure of the amount of energy (heat and light) shifted every second. The higher the power, the brighter the bulb.

Direct or indirect lighting: Do the lamps provide direct illumination to the room or is the light reflected off the wall or ceiling first?

• ## 04 Shallow poolsLi03TAnugget04 Activity

### Refraction can be deceiving

What the activity is for

This activity poses a problem that can be explained in terms of refraction.

Teacher: Why does the swimming pool look shallower than it really is?

What to prepare

• a means of sharing a large view of a computer screen
• this interactive presentation

What happens during this activity

This activity is an interactive teacher demonstration in which you introduce the problem of the swimming pool and talk through the explanation with the class.

Have you noticed that whenever you stand on the side of a swimming pool the water looks to be shallower than it really is?

When you dive in, you expect to be able to touch the bottom straight away, but it takes two or three strokes to get there.

Suppose you are standing on the side of the pool and you notice a red disc at the bottom. Once again it seems to be closer than it really is.

How can we explain this?

You need to imagine that you are leaning out over the pool looking down on the red disc.

A possible commentary to go with the presentation:

• First of all, if you are able to see the disc, then light must travel from the disc up through the water to your eye. Here we have drawn just two rays drawn up from the disc.
• Rays from the block meet the surface of the water. When the rays meet the surface, they do not carry straight on, but are refracted. Since the rays are passing from water to air (from a low speed medium to a high speed medium) they are refracted away from the normal.
• Rays appear to come from higher up in the water.
• The refracted rays leave the water. You are able to see the disc as rays from every point on its surface (not just the ones shown here) predict that beams will travel to your eye from all parts of the disc.
• Where do the rays predict that the light will appear to be coming from? The rays do not seem to be coming from the disc at the bottom of the pool but from a point somewhere above the bottom.
• This point can be found by extending the refracted rays back into the water. The depth at which the disc appears to be is referred to as the apparent depth of the pool.
• ## 05 How do lenses work?Li03TAnugget05 Activity

### Explaining lenses

What the activity is for

This activity shows how knowing what happens as light refracts allows an understanding of lenses.

What to prepare

• a means of sharing a large view of a computer screen
• this interactive presentation