Light is an electromagnetic wave (see page 66) and the straight line paths followed by very narrow beams of light, along which light energy travels, are called rays.
The behaviour of light rays may be investigated by using a ray-box. This consists merely of a lamp in a box containing a narrow slit which emits rays of light.
Light always travels in straight lines although its direction can be changed by reflection or refraction.
Figure 19.1 shows a ray of light, called the incident ray, striking a plane mirror at O, and making an angle i with the normal, which is a line drawn at right angles to the mirror at O.
i is called the angle of incidence. r is called the angle of reflection. There are two laws of reflection:
(i) The angle of incidence is equal to the angle of reflection (i.e. i D r in Figure 19.1)
(ii) The incident ray, the normal at the point of incidence and the reflected
ray all lie in the same plane.
A simple periscope arrangement is shown in Figure 19.2. A ray of light from O strikes a plane mirror at an angle of 45° at point P. Since from the laws
of reflection the angle of incidence i is equal to the angle of reflection r then i = r = 45°. Thus angle OPQ = 90° and the light is reflected through 90° . The ray then strikes another mirror at 45° at point Q. Thus a = b = 45° , angle PQR = 90° and the light ray is again reflected through 90°. Thus the light from O finally travels in the direction QR, which is parallel to OP, but displaced by the distance PQ. The arrangement thus acts as a periscope.
When a ray of light passes from one medium to another the light undergoes a change in direction. This displacement of light rays is called refraction.
Figure 19.3 shows the path of a ray of light as it passes through a parallel- sided glass block. The incident ray AB that has an angle of incidence i enters the glass block at B. The direction of the ray changes to BC such that the angle
r is less than angle i. r is called the angle of refraction. When the ray emerges from the glass at C the direction changes to CD, angle r0 being greater than i0 . The final emerging ray CD is parallel to the incident ray AB.
In general, when entering a more dense medium from a less dense medium, light is refracted towards the normal and when it passes from a dense to a less dense medium it is refracted away from the normal. Lenses are pieces of glass or other transparent material with a spherical surface on one or both sides. When light is passed through a lens it is refracted.
Lenses are used in spectacles, magnifying glasses and microscopes, telescopes, cameras and projectors.
There are a number of different shaped lenses and two of the most common are shown in Figure 19.4.
Figure 19.4(a) shows a bi-convex lens, so called since both its surfaces curve outwards. Figure 19.4(b) shows a bi-concave lens, so called since both
of its surfaces curve inwards. The line passing through the centre of curvature of the lens surface is called the principal axis.
Figure 19.5 shows a number of parallel rays of light passing through a bi-convex lens. They are seen to converge at a point F on the principal axis.
Figure 19.6 shows parallel rays of light passing through a bi-concave lens.
They are seen to diverge such that they appear to come from a point F, which lies between the source of light and the lens, on the principal axis.
In both Figure 19.5 and Figure 19.6, F is called the principal focus or the focal point, and the distance from F to the centre of the lens is called the focal length of the lens.
An image is the point from which reflected rays of light entering the eye appear to have originated. If the rays actually pass through the point then a real image is formed. Such images can be formed on a screen. Figure 19.7
illustrates how the eye collects rays from an object after reflection from a plane mirror. To the eye, the rays appear to come from behind the mirror and the eye sees what seems to be an image of the object as far behind the mirror as the object is in front. Such an image is called a virtual image and this type cannot be shown on a screen.
Lenses are important since they form images when an object is placed at an appropriate distance from the lens.
Bi-convex Lenses and their Applications
(i) Figure 19.8 shows an object O (a source of light) at a distance of more than twice the focal length from the lens. To determine the position and size of the image, two rays only are drawn, one parallel with the principal axis and the other passing through the centre of the lens. The image, I, produced is real, inverted (i.e. upside down), smaller than the object (i.e. diminished) and at a distance between one and two times the focal length from the lens. This arrangement is used in a camera.
(ii) Figure 19.9 shows an object O at a distance of twice the focal length from the lens. This arrangement is used in a photocopier.
(iii) Figure 19.10 shows an object O at a distance of between one and two focal lengths from the lens. The image I is real, inverted, magnified (i.e. greater than the object) and at a distance of more than twice the focal length from the lens. This arrangement is used in a projector
(iv) Figure 19.11 shows an object O at the focal length of the lens. After passing through the lens the rays are parallel. Thus the image I can be considered as being found at infinity and being real, inverted and very much magnified. This arrangement is used in a spotlight.
(v) Figure 19.12 shows an object O lying inside the focal length of the lens. The image I is virtual, since the rays of light only appear to come from it, is on the same side of the lens as the object, is upright and magnified. This arrangement is used in a magnifying glass.
For a bi-concave lens, as shown in Figure 19.13, the object O can be any distance from the lens and the image I formed is virtual, upright, diminished and is found on the same side of the lens as the object. This arrangement is used in some types of spectacles.
A compound microscope is able to give large magnification by the use of two (or more) lenses. An object O is placed outside the focal length Fo of a bi-convex lens, called the objective lens (since it is near to the object), as shown in Figure 19.14. This produces a real, inverted, magnified image I1.
This image then acts as the object for the eyepiece lens (i.e. the lens nearest the eye), and falls inside the focal length Fo of the lens. The eyepiece lens then produces a magnified, virtual, inverted image I2 as shown in Figure 19.14.
A simple projector arrangement is shown in Figure 19.15 and consists of a source of light and two-lens system. L is a brilliant source of light, such as a tungsten filament. One lens system, called the condenser (usually consisting of two converging lenses as shown), is used to produce an intense illumination of the object AB, which is a slide transparency or film. The second lens, called the projection lens, is used to form a magnified, real, upright image of the illuminated object on a distant screen CD.
