Class X, PHYSICS, "Propagation and Reflection of Light"

Definitions


1. Incident Ray

The ray that strikes the surface of the medium is known as Incident Ray.


2. Reflected Ray

The ray that is sent back into the same medium after reflection is known as reflected ray.

3. Plane Mirror


A flat smooth reflecting surface, which shows regular reflection is known as plane mirror.

4. Normal

Perpendicular line on the reflecting surface is known as normal.

5. Pole

The centre of the spherical mirror is called pole.

6. Angle of Incidence

The angle subtended by the incident ray to the normal is known as angle of incidence.

7. Angle of Reflection

The angle subtended by the reflected ray to the normal is known as angle of reflection.

8. Center of Reflection

The center of the hollow sphere of which the mirror is a part is called center of curvature.

9. Principle Axis

The straight line passing through center of curvature nad the pole is known as principle axis.

10. Principle Focus

The ray coming parallel to principal axis after converges to or diverges from a point, which is called principle focus.

11. Focal Length

The distance between the principle focus and pole of the mirror is called Focal Length.

12. Radius of Curvature

The distance between the center of curvature and the pole is called radius of curvature.

13. Real Image

The image that can be seen on a screen is known as a real image.

14. Virtual Image

The image that cannot be seen on a screen is known as a virtual image.

15. Magnification

The ratio between the image height and object height is known as magnification.

or

The ratio between the image distance to the object distance is known as magnification.

Reflection of Light

Definition

“The process in which light striking the surface of another medium bounces back in the same medium is known as Reflection of Light.”

Laws of Reflection

1. The angle of reflection, is equal to the angle of incidence: n 2. The incident ray, reflected ray and normal, all lie in the same plane.

Kinds of Reflection

There are two types of Reflection:

1. Regular Reflection

Definition

When parallel rays of light strike a surface and most of them are reflected in a same particular direction or same angle, they are said to be regularly reflected and the phenomenon is known as regular reflection.

Regular reflection occurs when parallel rays of light strike with an ideal smooth plane surface. In regular reflection parallel rays remain parallel after reflection.




2. Irregular Reflection

Definition

When some rays of light strikes a surface and the reflected rays scatter in different directions, this type of reflection is called irregular reflection.

It occurs when parallel rays strike with an irregular rough surface. In this case rays does not remain parallel after reflection and they scattered.



Advantages of Irregular Reflection

* Due to this reflection, sunlight reaches us before sunrise and persists for some time even after the sunset.
* Due to this reflection we get sufficient light in our rooms and other places where sunlight do not reach directly.
* Due to this reflection sunlight reaches to each of the leaves of a tree and photosynthesis takes place on large scale.
* Due to this reflection, we can see luminous objects.

Image Formed by a Plane Mirror

Consider a mirror MM’, AP is an object. Consider that a point P lies on the tip of the object. From P as ray travels and strikes mirror and reflect back to the eye, they appear to come back. From Point P’ as shown in the figure. Hence P’ is the image of P. Similarly, infinite points lying an object produces infinite images of points and complete image of an object is formed.



Characteristics of Image Formed by a Plane Mirror

* Image is same in size as that of the object.
* The distance of object and image are equal from the mirror.
* The image formed is virtual and inverted.

Spherical Mirrors

Definition

“A spherical mirror is a section of a of a hollow sphere.”



Types of Spherical Mirrors

There are two types of spherical mirror:
1. Concave Mirror (Converging Mirror)
2. Convex Mirror (Diverging Mirror)

1. Concave Mirror

Definition

“The spherical mirror in which inner side of the surface is polished for reflection is called a concave mirror.”


Properties

* The bulging side is polished.
* Reflection occurs from its hollow side.
* They converge the parallel rays at a point.
* They can form real and imaginary, both types of images.

2. Convex Mirror

Definition
“The spherical mirror in which inner side of the surface is polished for reflection is called concave mirror.”

Properties

* The bulging side is polished.
* Reflection occurs from its hollow side.
* They converge the parallel rays at a point.
* They can form real and imaginary, both type of images.

Formation of Image by Concave Mirrors

There are six cases to form an image by concave mirror.

1. Object at Infinity


If the object is placed at infinity from the mirror, the rays coming from the object are parallel to principal axis. After reflection, they meet at principal focus and image is formed at the focus.

Details of Image

* Formed at F.
* Extremely Small
* Real
* Inverted

2. Object Beyond C


If the object is placed beyond C, rays coming from the object are not parallel. They meet after reflection between the focus and center of curvature. Therefore, image is formed between the focus and center of curvature.

Details of Image

* Formed between F and C.
* Small in size.
* Real
* Inverted

3. Object at Center of Curvature ‘C’

When object is placed at the centre of curvature, the image formed at the same place.


Details of Image

* Formed at C
* Equal in size
* Real
* Inverted

4. Object Between F and C


When the object is placed between the focus and Centre of curvature, the image is formed beyond the centre of curvature.

Details of Image

* Formed beyond C.
* Large in size.
* Real
* Inverted

5. Object at F


When object is placed at focus the reflected rays become parallel to each other. The two parallel lines meet at infinity. Therefore, we say the image is formed at infinity.

Details of Image

* Formed at Infinity.
* Extremely Large
* Real
* Inverted

6. Object between P and F


For locating object between pole and focus the rays reflected do not meet because they diverge. But they meet backward. So, the image is formed backward or behind the mirror.

Details of Image

* Formed behind the mirror.
* Large in size
* Virtual
* Erect

Uses of Spherical Mirror

Spherical mirrors are used in several places. Some of them are given below:
Shaving: A concave mirror is used to enlarge the image.
Microscope: A convex mirror is used for magnification in a microscope.
Telescope: The convex mirror is used.
In Searchlights and Headlights: Concave mirror is used to form the rays in searchlights and headlights, used for different purposes.
For Rear View: The convex mirror is used in automobiles.
In Medical Examination (Opthalmoscope): Doctors use concave mirror for the examination of ear, nose, throat and eyes of patients.