Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Real and virtual images in a smoke box

Demonstration

Shows rays forming an image in the smoke with a lens and a mirror, as well as the effects of chromatic and spherical aberration.

Apparatus and materials

Smoke box

Smoke generator or bee-smoker

Corrugated cardboard to burn

Plano-convex lens, large (100 mm approx)

Concave mirror, large

Lens holder

Aluminium screen with one large hole (50 mm approx)

Aluminium screen with 50 mm circle of holes

Aluminium screen with 75 mm circle of holes

Light source, compact (100 W 12 V)

L.T. variable voltage supply (12 V 8 A)

Retort stand and boss

Health & Safety and Technical notes


Be aware that compact light sources using tungsten-halogen lamps without filters are significant sources of UV. Ensure that no-one can look directly at the lamp. 
 
The method of making smoke with a bee-smoker is safe. Do not be tempted to generate smoke with chemicals.

1 You can use the smoke box for a number of demonstrations. If you cannot buy one, construction details are given in the Smoke box apparatus listed above. 

Good smoke can be made by rolling up corrugated cardboard, lighting it and inserting the lit end into the bottom of a bee-smoker. Take care not to extinguish the flame too quickly by pushing the bellows too hard to begin with. 
 
2 The plano-convex lens should be of condenser quality, about 100 mm in diameter, 150 mm focal length. A suitable lens is obtainable from Knight Optical (UK) Ltd, part number LPC15050. 
 
3 The large concave mirror can be one of the cheap plastic mirrors often supplied in multiple packs, although these only have a focal length of about 40 mm. 
 
4 The aluminium screen has a series of small holes drilled in a circle of about 50 mm diameter. This produces a cone of rays when a light is shone at it. A second screen with the holes in a circle of about 75 mm is useful too. 
 
5 The laboratory needs to be quite dark for the smoke to be seen clearly. 
 
6 Smoke will slowly escape from the box and will need replacing regularly, but there really is no need for coughing fits from disruptive students! So warn them! 
 
7 Clean the lens and mirror before putting them away. 
 

Apparatus seup

 

Procedure


a Put the lens inside the box, about 100 mm from the end, with its convex side towards the lamp. This will reduce aberration. 

b Attach the aluminium sheet with holes to the end of the box. 
 
c Light a piece of corrugated cardboard with a match. When it is burning, insert it in the smoke generator. Insert the generator through the opening in the back of the smoke box. Several strokes of a bee smoker should fill the box. 
 
d Place the compact light source about 1/2 a metre from the lens, outside the smoke box. 
 
e Bring the lamp nearer to the lens and observe how the image changes. 
 
f Take away the screen with holes and replace it with the screen with one large hole. This is to show the effect of the lens on the beam of light and, less vividly, the dark shadow produced by the lens. Observe the aberrations. 
 
g Remove the screen with the large hole and insert the screen with the smallest circle of holes. Move the lens close to the entrance window, this time with the convex side of the lens facing away from the lamp, to minimize aberration. Place the lamp near the window. To show rays diverging from a virtual image, the lamp must be at most 120 mm from the lens. Fill the box with smoke as before. 

inside Smoke box
  
h Remove the lens from the box. Return the lamp to its original position about half a metre from the front of the box, with the aluminium screen with the 50 mm circle of holes in position. Place the concave mirror in the box and fill it with smoke. Direct a fan of rays at the mirror.


Teaching notes


1 The lamp and the lens should be placed so that the rays cover the full aperture of the lens. Rays which fall outside the lens aperture carry on in a straight line, providing a useful lesson about rays of light. 

When you first inject smoke into the box it will swirl around inside. When it settles, a beautiful 3D fan of rays will be revealed. These rays are bent by the lens to form an image in space, with the rays passing straight through the image. The smoke in the smoke box scatters some of the light so that the rays can be seen. 
 
Students will have to walk round the box looking at the rays from the side and near end-on to get the best effect. (Although rays can be seen best by observers looking 'upstream' towards the light source, it is unwise to do this because of the UV content.) 
 
2 When the lamp is brought nearer to the lens, in e, the image moves further away. 
 
3 Here are two kinds of lens defect, or aberration: 

  • The screen with a large hole will produce a bright cone, with coloured edges. The red rays are brought to a different focus from the blue ones. This is called chromatic aberration. 
  • The image will be fuzzy because outer rays of the cone will not be brought to the same focus as more paraxial rays. This is called spherical aberration. 

 
4 When the lens and the light source are close together, as in g, the rays splay out so strongly that the lens cannot bend them enough to form a real image. However, it does bend them so that the rays seem to come from a virtual image behind the lamp. This is how a magnifying glass behaves. 
 
5 Using a concave mirror as in h, is another way to produce a real image. This time there is no chromatic aberration. Point out that this gives reflecting telescopes an important advantage over refracting telescopes. 
 
6 The smoke box can be seen as a pinhole camera, cut along its length. You can see what the lens is doing to the rays of light. The holes in the screen represent the holes in the pinhole camera. 
 
This experiment was safety-checked in December 2006