Reflection – Laws, Types, and Applications

Reflection is a phenomenon that we experience in our daily lives when we use a mirror. The reflection of light from various kinds of surfaces allows us to see our surroundings. The point at which the rays of light emanating from an object or optical source appear to meet after reflection from the mirror forms an image. In this article, let us know what reflection is and what its types are.

Reflection Definition

The change in the direction of a wave when it hits a boundary between two different media and moves back into the same medium is called reflection. The commonly known reflections are that of light and sound waves.

Reflection of Light

Light reflects from any surface according to two rules. These rules are called laws of reflection. When light from any object reflects from polished surfaces, reflection is called regular reflection. Regular reflection gives images. Depending on the mirror (reflecting surface) and object position we get two types of images called real and virtual images.
Plane mirrors we use at home form our virtual image. Light rays falling on it get bounced back by the smooth surface, and we can see the reflection of the object in the mirror. For the image formed by a plane mirror, the image distance from the mirror will be equal to the object distance from the mirror.

We can see that the image distance from the mirror is equal to the distance of the object from the mirror.

\({d_o} = {d_i}\)

Laws of Reflection

Incident ray: The ray of light that hits the reflecting surface is the incident ray.
Reflected ray: It is the ray that bounces back from the surface.
Normal: It is the line that is perpendicular to a surface.
Point of incidence: It is the point where the incident ray touches the surface.

1. First Law

The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.
Suppose we imagine a plane passing through the incident ray and the reflected ray. In that case, it will also contain the normal to the surface at the point of incidence.

2. Second Law

The angle of incidence is equal to the angle of reflection.
In simple words, if the angle of incidence is \({60^ \circ }\) to the normal, then the ray reflects or turns back at the same angle of \({60^ \circ }\) to the normal.

Image

The rays coming from a point (part of an object) after interacting with an optical medium, either converge to a point or appear to diverge from a point. If they converge the point to which they are converging is called a real image, and if they diverge, the point from which they are appearing to diverge is called a virtual image.

Real Image: Rays from the object after interacting with the optical medium like mirror or lens, converge to a point to form a real image. A real image can be taken on-screen if we place a screen at the place where rays are converging.

Virtual Image: Rays from the object after interacting with the optical medium like mirror or lens, appear to diverge from a point. The point from which they are appearing to diverge is called a virtual image. We cannot take a virtual image on the screen.

Types of Reflection

Based on the type of surface to which light hits, there are three types of reflections:
a. Regular reflection
b. Diffused reflection
c. Multiple reflections

a. Regular Reflection

Regular reflection is the reflection from a smooth surface such as a mirror, polished metal, or still surface of the water. Another name for this is Specular reflection. The image obtained is clear and sharp. If a parallel beam of light is incident on the surface, all the reflected rays are also parallel. Thus, a regular reflection forms an image.

The image formed in a flat, plane mirror is virtual, erect, and the same size as that of the object.
We can only see a virtual image but cannot project it on a screen.

b. Diffused Reflection

A diffused reflection is caused due to light falling on rough surfaces. A beam of light falling on such a surface gets reflected in all directions at various angles. When light falls on the objects in our room, it gets diffused in all directions based on the roughness of their surfaces.

Images are not formed in diffused reflection.

C. Multiple Reflections

Multiple reflections are like regular reflections, but with more than one mirror. When we place two mirrors at an angle, multiple reflections take place in such cases. The number of images seen depends on the angle between the two mirrors.
The method to determine the number of images is as below:

Step 1:

Find the ratio of \(360^\circ \) to the angle between mirrors.
\(n = \frac{{{{360}^o}}}{{{\rm{angle\;between\;mirrors}}}}\)

Step 2:

If \(n\) is an even number, then
Number of images \(\left( N \right) = n – 1\)
If \(n\) is an odd number, then,
a)If object is kept symmetrically, Number of images \(\left( N \right) = n – 1\)
b)If object is kept asymmetrically, Number of images \(\left( N \right) = n\)
If \(n\) is a fraction then number of images \(N\) is equal to integral part.

Example 1:

If we place two mirrors at an angle of \({60^ \circ },\) then we get \(5\) images as \(\frac{{360}}{{60}} = 6,\) which is an even number.

Example2 :

Suppose if we place two mirrors at an angle of \({120^ \circ }\) and object is kept symmetrically, Since \(\frac{{360}}{{120}} = 3,\) we see \(3 – 1 = 2\) images.

Example 3:

In a room with mirrors on opposite walls, such as a haircutting salon, the angle between them is zero. This gives rise to infinite reflections, as \(\frac{{360}}{{0}} = \infty \)
We use curved mirrors also in many situations. Two main types of curved mirrors are concave mirrors and convex mirrors.

Reflection Due to Concave Mirror

A concave mirror is a part of a sphere whose inside surface is smooth and shiny. A concave mirror can give an erect, virtual image or a real, inverted image depending on the object’s position.

We can project a real image on a screen.

Real Image	Virtual Image

Uses of Concave Mirrors

i. Men use them while shaving.
ii. They are used to produce parallel beams of light in torches, vehicle headlights, searchlights.
iii. They are used in reflecting telescopes.
iv. Dentists use them to see the back of teeth.

Reflection Due to Convex Mirror

A convex mirror is a part of a sphere whose outside surface is smooth and shiny. Irrespective of the object’s position, this mirror always gives a virtual, erect, and diminished image.

Uses of Convex Mirrors

i. Vehicle side view mirrors are convex as they show a larger area in a small mirror.
ii. They are placed at curves of roads to give a view of the bend of the road.
iii. Convex mirrors are also used in reflecting telescopes.

Other Types of Reflection

Water waves, sound waves, seismic waves also get reflected.

Reflection of Sound

Echo is the most common experience of sound reflection. When we stand near a hill and shout, we can hear our voice after a few seconds, due to echoing. All sounds get reflected by the walls and objects in our surroundings. However, we do not hear echoes in our houses or buildings as the distance of travel of sound waves is too small.
Reflection of sound has applications in devices like SONAR (Sound Navigation and Ranging) to detect objects and calculate the ocean’s distances and depths. Echocardiography is used to diagnose heart problems.
Bats and dolphins use echoes to move about and find food.

Reflection of Seismic Waves

Seismic waves from earthquakes and explosions too get reflected by different layers of the earth. This has application in studying the earth’s crust and in prospecting for petroleum and gas.

Applications of Reflection

a. We use plane mirrors and spherical mirrors which work on laws of reflections, in our daily lives in different situations.
b. Sound reflection is used to find depths of oceans and in medical devices like echocardiography.
c. Seismic wave reflection is used to determine the presence of petroleum deposits.

Difference Between Real Image and Virtual Image

Following are the differences between real image and virtual image:

(i) When the rays of light emanating from an optical source meet at the real point after reflection from the mirror, that point source is called a real image. Whereas, when the rays of light emanating from an optical source appear to meet after reflection from the mirror, it is called a virtual image of that point source.

(ii) Real images can be obtained on a screen, whereas virtual images cannot be obtained on a screen.

(iii) The real image is always inverted as compared to the object. On the other hand, the virtual image is always erect as compared to the object.

(iv) Real image is represented by real lines, whereas the virtual image is always represented by dots or by broken lines.

Sample Problems

Q.1. Two plane mirrors are placed such that they make an angle of \({90^ \circ }.\) How many reflections of an object can be seen?
Ans: \(n = \frac{{{{360}^ \circ }}}{{{\text{angle}}\,{\text{between}}\,{\text{mirrors}}}}\)
\(n = \frac{{{{360}^ \circ }}}{{{{90}^ \circ }}} = 4\)
Since \(n\) is an even number, the number of images seen is \(n – 1 = 3\)
We see three images when we place two mirrors perpendicular to each other.

FAQs

Q.1. What is reflection in simple words?
Ans: Reflection means the bouncing of a wave when it hits the surface of an object. It can be light, sound, water waves, radio waves, seismic waves.

Q.2. Why do we see diminished or distorted images on the outer surface of steel vessels?
Ans: Steel vessels are smooth. Since they are curved, the outer surface behaves like a convex mirror.

Q.3. Where do we experience the reflection of sound?
Ans: We experience the reflection of sound near hills, large buildings, inside large halls. It is called echo.

Q.4. What type of reflection makes it possible for us to see our surroundings?
Ans: Our eyes receive light from all directions due to diffused reflection from objects when light falls on them, and it gets scattered.

Q.5. In reflection of light, what is the difference between real and virtual images?
Ans: A real image is inverted and can be projected on a screen. It can be captured on a light-sensitive film or sensor. A virtual image is erect but cannot be projected on a screen. Likewise, it cannot be captured on a photographic film or sensor.

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