Light- Reflection and Refraction Class 10th Notes- Free NCERT Class 10 Science Chapter 10 Notes - Download PDF

Light Reflection And Refraction Class 10 Notes: Download PDF

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NCERT Notes for Class 10 Science Chapter 9

Reflection Of Light

Reflection is defined as the bouncing back of light rays into the same medium when these rays strikes on a surface or on a boundary separating two media.

Laws of Reflection:

• The angle of incidence is equal to the angle of reflection.

• The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.

Spherical Mirrors:

Consider a hollow sphere with a highly polished internal surface and a mercury-coated exterior surface that prevents light from passing through. After that, we can cut a thin slice out of the shell to make a curved mirror known as a spherical mirror.

Concave Mirror: A concave mirror is a spherical mirror with a reflecting surface that is curved inwards, towards the center of the sphere.

Convex Mirror: A convex mirror is a spherical mirror with a curved outwards reflecting surface.

When studying spherical mirrors, there are a few essential terms to be aware of.

Principal axis: The imaginary line that passes through the optical centre and curvature centre of any lens or spherical mirror.

Centre of Curvature: The point in the mirror's centre that passes through its curve and has the same tangent and curvature as the curve.

Radius of Curvature: The linear distance between the pole and the centre of curvature is known as the radius of curvature.

Pole: It is the midpoint of the spherical mirror.

Principal focus: Focal Point is another word for the Principal Focus. After reflection or refraction on the axis of a mirror or lens occurs, it is the point at which light rays parallel to the axis converge or appear to converge.

Focus: It is any point at which light rays parallel to the principal axis converge after being reflected by the mirror.

Focal length: The distance between the pole and the principal focus is the focal length of a spherical mirror.

Aperture: An aperture in a mirror or lens is the point at which light is reflected. It also specifies the size of the mirror.

The radius of curvature of small aperture spherical mirrors is found to be twice the focal length. This is denoted by R = 2f. This means that the principal focus of a spherical mirror is located midway between the pole and the centre of curvature.

Image Formation by Spherical Mirrors:

Representation of Images Formed by Spherical Mirrors Using Ray Diagrams

• A ray of light that converges or diverges from the focus after reflection and is parallel to the major axis of a spherical mirror.
  

• A light ray travelling through or appearing from the spherical mirror's centre of curvature is reflected back along the same path.
  

• A light ray travelling through or coming from the focus of a spherical mirror becomes parallel to the principal axis.
  

• A ray of light incident at a spherical mirror's pole is reflected back making the same angle at the principal axis.
  

Image formation by a concave mirror:

Uses of Concave Mirrors:

To obtain intense parallel beams of light, concave mirrors are often employed in torches, searchlights, and vehicle headlights.

They're frequently utilized as shaving mirrors to get a better view of one's face. Concave mirrors are used by dentists to see big images of patients' teeth.

Solar furnaces use large concave mirrors to focus sunlight and produce heat.

Image Formation by Convex Mirror

Uses of Convex Mirrors

Vehicles frequently employ convex mirrors as rear-view (wing) mirrors. These side mirrors are mounted on the vehicle's flanks and allow the driver to observe traffic behind him or her, ensuring safer driving. Convex mirrors are preferred because they produce an erect if diminished, picture every time. Furthermore, because they are curved outwards, they have a larger field of view. As a result, convex mirrors allow the driver to see a considerably wider area than a plane mirror would

Sign Convention for Reflection by Spherical Mirrors

All distances are measured from the origin, which is the pole of the mirror.

• Positive distances are those measured in the direction of incident rays.

• The lengths measured in the opposite direction of the incoming rays are known as negative distances.

• Positive distances are those measured upward and perpendicular to the principal axis.

• Distances measured below and perpendicular to the principal axis are considered negative.

Mirror Formula and Magnification

The distance between an object and its pole in a spherical mirror is known as the object distance (u). The image distance is the distance between the picture and the pole of the mirror (v). The focal length (f) is the distance between the primary focus and the pole. The mirror formula provides a relationship between these three quantities, which is stated as

$\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$

Magnification:

Magnification ( $m$ ) of a spherical mirror is defined as the ratio of the height of the image to the height of the object. Its magnitude indicates how many times the size of the image is as compared to that of the object and its sign indicates whether it is erect or inverted. Positive sign indicates an erect image and negative sign indicates an inverted image.

$
m=\frac{\text { height of the image }}{\text { height of the object }}=\left[\frac{h^{\prime}}{h}\right]=-\frac{v}{u}
$

Refraction of Light

The refraction of light is the bending of light beams as they move from one medium to another.

Laws of Refraction

The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.

The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.

If i is the angle of incidence and r is the angle of refraction, then,

$\frac{\sin i}{\sin r}$ constant

The Refractive Index:

The refractive index expresses the magnitude of the change in light direction that occurs in a given pair of media. This can be stated mathematically as,

$\mathrm{n}_{21}= \frac{\text{Speed of light in medium 1}}{\text{Speed of light in medium 2}} =\frac{\mathrm{v}_1}{\mathrm{v}_2}$

Similarly, the refractive index of medium 1 in comparison to medium 2 is denoted as $n_{12}$.

If medium 1 is vacuum or air, the refractive index of medium 2 with respect to vacuum is taken into account. This is referred to as the medium's absolute refractive index.

Refraction by Spherical Lenses

Lens: A spherical lens is a piece of transparent glass joined by two spherical surfaces.

Spherical lenses are classified into two categories.

Convex lens: A convex lens bulges outward and is thicker in the centre and narrower at the corners. As depicted below, a convex lens converges the light rays. As a result, convex lenses are called converging lenses.

Concave lens: A concave lens bulges inward and is thinner in the centre and thicker at the borders. As seen in Figure below, such lenses diverge light rays.

A lens, whether convex or concave, has two spherical surfaces that form a sphere. The centres of these spheres are known as lens' centres of curvature, which are generally denoted by the letter C. Because there are two centres of curvature, we can refer to them as C1 and C2.

The principal axis of a lens is an imaginary straight line that passes across its two centres of curvature, as shown in Figure (a).

The optical center of a lens is located at its central point. It is commonly represented by the letter O. A ray of light travels through the optical centre of a lens without deviating.

The effective diameter of a spherical lens's circular contour is referred to as its aperture.

Figure (a) shows numerous beams of light parallel to the primary axis falling on a convex lens. These rays are converging to a point on the primary axis after being refracted by the lens. This position on the major axis is referred to as the lens's principal focus.

The letter F is commonly used to denote principal focus. A lens has two principal foci.

Similarly, in Figure (b), numerous beams of light parallel to the principal axis fall on a concave lens. These rays appear to diverge from a point on the principal axis after being refracted by the lens. The principal focus of the concave lens is located on the principal axis.

The focal length of a lens is the distance between the principal focus and the optical centre, denoted by the letter f.

Image Formation by Lenses

Nature, position and relative size of the image formed by a convex lens for various positions of the object

Nature, position and relative size of the image formed by a concave lens for various positions of the object

Image Formation in Lenses Using Ray Diagrams:

The ray diagram allows us to investigate the nature, position, and relative size of the picture created by lenses.

For Convex lens:

For Concave lens:

The ray diagrams for image generation in a convex lens for a few object placements are presented in the table below.

The ray diagrams for image generation in a concave lens for a few object placements are presented in the table below.

Sign Convention for Spherical Lenses

• All distances are measured from the optical centre of the lens.

• Positive distances are those measured in the same direction as the incident light.

• Negative distances are those measured in the direction of incident light.

• Positive distances are those measured upward and perpendicular to the principal axis.

• Distances measured downhill and perpendicular to the principal axis are considered negative.

Lens Formula and Magnification

We also have a spherical lens formula. This equation expresses the relationship between object distance (u), image distance (v), and focal length (f). The lens formula is written as

$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

The letter M stands for magnification. If h is the height of the object and h′ is the height of the image generated by a lens, then the lens's magnification is given by,

$m=$ Height of image(h') / Height of Object (h)

$\mathrm{m}=\mathrm{h'} / \mathrm{h}$

Power of a Lens

A lens's power is defined as the reciprocal of its focal length. It is denoted by the letter P. The power P of the focal length f of a lens is given by,

$P=\frac{1}{f}$

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