There are several ways to think about light in physics. One very useful way is
to think of it in terms of rays. That is, to imagine light to be traveling in very
narrow beams. When you do that, we say that you are modeling light as rays. This
method allows one to develop an understanding of several light phenomena including
common reflections and refractions.
An image in which the rays of light radiating from a point on the object converge
to a physical point in space is called a real image. Parallel to this is the concept
of a virtual image. When the rays from an object point never actually converge towards
a different physical point in space but simply appear to the eye as if they were
radiating from an image point, we call the image a virtual image.
Ray Optics covers a large variety of topics like the refractive index, mirror
formula, lens formula, refraction at spherical surfaces, refraction through prism
etc. We shall give a brief outline of these topics as they have been discussed in
detail in the coming sections:
Reflection is a simple yet important concept. Almost all objects in the world
reflect a certain amount of light falling on them. When light falls on the object,
it gets reflected and this is the color that is visible to the human eye. The ray
that falls on the surface is termed as the incident ray and the angle that it makes
with the normal is called the incident angle. The normal is the imaginary line that
is perpendicular to the surface at the point where it is intersected by the incident
ray. This ray again springs back as the reflected ray and it has the same angle
of reflection as the angle of incidence from the normal.
Hence, from this discussion, we obtain the law of reflection which states that
Angle of incidence = Angle of reflection
One point to be noted here is that all these lines including the incident ray,
the reflected ray and the normal to the surface lie in the same plan. Reflection
can broadly be categorized as Specular Reflection and Diffuse Reflection.
Specular Reflection: When light rays which are in
the form of parallel lines strike against a smooth or a plane surface and then get
reflected again in the form of parallel lines, then this form of reflection is called
as the specular reflection. The following figure will prove useful in furthering
clearing the concept of this kind of reflection:
Diffuse Reflection: When light rays fall in the
form of parallel lines on a rough surface and as result they get reflected in all
directions, such type of distortion is termed as diffuse reflection.
The concept of refraction and the index of refraction are of immense importance.
If light travels from point A to point B, then its speed will be highest if it travels
in a straight line. But it has to pass through various materials then it will pass
through them at different speeds and the motion will not be in a straight line.
Hence, when it enters a new medium, it gets bent a bit and this bending is termed
as refraction. The human eye always assumes light to be traveling in a straight
line and hence when an object appears to be bent slightly due to refraction, we
assume that the object is bent and not the light.
The index of refraction depends on the medium through which light passes. The speed
of light is more in medium which are less optically dense and less in more optically
dense mediums. The mathematical formula for calculation of index of refraction is:
n = Speed of light in vacuum/ Speed of light in the medium = c/v
The index of refraction is represented by the letter ‘n’ and denotes the angle at
which the light bends. When light travels from a medium with refractive index n1
to the other with refractive index n2, the relation between the angle of incidence
θ1 and the angle of refraction θ2 is given by
n1sin θ1 = n2 sin θ2
Since the speed of light in air is almost the same as the speed of light in a vacuum,
so in most of the cases a value of one for the index of refraction of air. We have
listed the indices of refraction for various substances in the table given below:
Substance (at 20° C)
Index of refraction, n
Refraction through a glass prism:
When the visible white light passes through an equilateral prism, it experiences
dispersion. It was proposed by Newton that it was possible to divide the white light
into its various component colors with the help of an isosceles prism having equal
sides and angles. As soon as the light ray falls on the surface of a dispersing
prism, on entering it gets refracted and then passes through the glass unless it
reaches the second boundary. Again the light gets refracted then it follows a new
path on exiting. When the waves pass through prism, they get deviated by a certain
angle which can be calculated. We obtain the angle of minimum deviation when the
angle at which the light wave enters the prism permits the beam to pass through
the glass in a parallel direction to the base.
As the values of the refractive index of a prism are increased, it also leads
to an increase in the angle of deviation of light passing through prism. Refractive
index is also affected by wavelength of light. The shorter wavelengths are refracted
at greater angles while the longer wavelengths like red light are refracted at small
angles. This variation in the angle of deviation in prism is termed as dispersion.
Ray optics discusses topics like refraction of light through prism or the angle
of prism and it is quite different form Wave Optics. Ray theory does not describe
phenomena such as interference and diffraction, which require wave theory (involving
the phase of the wave).
Deviation and refraction are two concepts which are quite inter-related and often
confused with each other. But there is a difference between the two. When light
enters a different medium, the change that occurs in its path like bending or turning
is termed as refraction. Deviation refers to the amount of this deflection in the
path of light when it enters some other medium.
Difference between Deviation and Refraction