10th Class Physics Chapter 9: Light Reflection and Refraction

Reflection of Light

• Light is reflected by objects and allows us to see them.
• The angle of incidence is equal to the angle of reflection, and the incident ray, normal to the mirror, and reflected ray all lie in the same plane.
• These laws of reflection are applicable to all types of reflecting surfaces, including spherical surfaces.

Image Formation by Plane Mirrors

• Image formed by a plane mirror is always virtual and erect.
• The size of the image is equal to that of the object.
• The image formed is as far behind the mirror as the object is in front of it.
• The image is laterally inverted.

Spherical Mirrors

• A spherical mirror's reflecting surface is curved inwards or outwards.
• A spherical mirror with a reflecting surface curved inwards is called a concave mirror.
• A spherical mirror with a reflecting surface curved outwards is called a convex mirror.
• The centre of the reflecting surface of a spherical mirror is called the pole.
• The centre of curvature of a spherical mirror is the point on the sphere that the mirror is a part of.
• The radius of curvature of a mirror is the radius of the sphere that the mirror is a part of.

Concave Mirrors

• The centre of curvature of a concave mirror lies in front of it.
• The principal focus of a concave mirror is the point where parallel rays converge after reflection.
• The distance between the pole and the principal focus of a concave mirror is called the focal length.
• The principal focus of a concave mirror lies midway between the pole and the centre of curvature.

Convex Mirrors

• The centre of curvature of a convex mirror lies behind the mirror.
• The principal focus of a convex mirror is the point where parallel rays appear to diverge from after reflection.
• The principal focus of a convex mirror lies midway between the pole and the centre of curvature.

Image Formation by Spherical Mirrors

• The image formed by a spherical mirror can be real or virtual, depending on the position of the object.
• The image can be magnified, reduced, or have the same size as the object, depending on the position of the object.
• The image formed by a concave mirror can be real and inverted, or virtual and erect, depending on the position of the object.

Ray Diagrams

• Ray diagrams can be used to study the formation of images by spherical mirrors.
• An infinite number of rays originate from each point of an extended object.
• Two rays can be considered to locate the image of an object, and the intersection of at least two reflected rays gives the position of the image.### Spherical Mirrors
• A ray parallel to the principal axis, after reflection, will pass through the principal focus in case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror.
• A ray passing through the principal focus of a concave mirror or a ray directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis.
• A ray passing through the centre of curvature of a concave mirror or directed towards the centre of curvature of a convex mirror, after reflection, is reflected back along the same path.
• A ray incident obliquely to the principal axis, towards the pole of the mirror, is reflected obliquely.

Image Formation by Concave Mirrors

• The laws of reflection are followed at the point of incidence.
• The angle of reflection equals the angle of incidence.
• The ray diagrams for the formation of image by a concave mirror for various positions of the object are shown in Fig. 9.7.

Uses of Concave Mirrors

• Concave mirrors are commonly used in torches, search-lights, and vehicles' headlights to get powerful parallel beams of light.
• They are often used as shaving mirrors to see a larger image of the face.
• Dentists use concave mirrors to see large images of the teeth of patients.
• Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.

Image Formation by Convex Mirrors

• The ray diagrams for the formation of image by a convex mirror for various positions of the object are shown in Fig. 9.8.
• The image formed by a convex mirror is virtual, erect, and diminished.

Uses of Convex Mirrors

• Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles.
• They are preferred because they always give an erect, though diminished, image.
• They have a wider field of view as they are curved outwards.

Sign Convention for Reflection by Spherical Mirrors

• The New Cartesian Sign Convention is used to deal with the reflection of light by spherical mirrors.
• The conventions are:
  • The object is always placed to the left of the mirror.
  • All distances parallel to the principal axis are measured from the pole of the mirror.
  • All the distances measured to the right of the origin (along + x-axis) are taken as positive while those measured to the left of the origin (along – x-axis) are taken as negative.
  • Distances measured perpendicular to and above the principal axis (along + y-axis) are taken as positive.
  • Distances measured perpendicular to and below the principal axis (along – y-axis) are taken as negative.

Mirror Formula and Magnification

• The mirror formula is 1/v + 1/u = 1/f, where v is the image distance, u is the object distance, and f is the focal length.
• Magnification produced by a spherical mirror gives the relative extent to which the image of an object is magnified with respect to the object size.
• Magnification is expressed as the ratio of the height of the image to the height of the object.
• Magnification is also related to the object distance and image distance.

Refraction of Light

• Light seems to travel along straight-line paths in a transparent medium.

• When light enters from one transparent medium to another, it changes its direction.

• Day-to-day experiences, such as the bottom of a tank or a pond containing water appearing to be raised, or a pencil partly immersed in water appearing to be displaced, can be explained by refraction of light.### Refraction of Light

• Refraction is the bending of light as it passes from one medium to another.

• The extent of refraction depends on the pair of media and the angle of incidence.

• A pencil appears to be displaced when viewed from above and below the water surface.

• The appearance of the pencil's displacement varies with the type of liquid used (e.g., water, kerosene, or turpentine).

Activity 9.7

• Place a coin at the bottom of a bucket filled with water.
• Try to pick up the coin from the side of the bucket.
• Repeat the activity to observe the effect of refraction.

Activity 9.8

• Place a coin in a shallow bowl and move away until the coin disappears from view.
• Ask a friend to pour water into the bowl without disturbing the coin.
• Observe how the coin becomes visible again due to refraction.

Activity 9.9

• Draw a straight line on a sheet of paper and place a glass slab on top of it.
• Observe how the line appears to be bent at the edges of the glass slab.
• Move the glass slab to make it normal to the line and observe the change.

Refraction through a Rectangular Glass Slab

• Perform an activity to study the refraction of light through a glass slab.
• Observe how the light ray changes direction at the points of incidence and emergence.

Laws of Refraction

• The incident ray, refracted ray, and normal to the interface of two media lie in the same plane.
• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given pair of media and a given color of light (Snell's law).

Refractive Index

• The refractive index is a measure of the extent of refraction.
• It is the ratio of the speed of light in the first medium to the speed of light in the second medium.
• The refractive index can be expressed as n21 = v1 / v2.

Absolute Refractive Index

• The absolute refractive index of a medium is its refractive index with respect to vacuum.
• It is represented by the symbol n.

Refractive Index of Various Media

• The refractive indices of various media, such as air, water, glass, and diamond, are given in Table 9.3.

Optical Density

• Optical density is a measure of a medium's ability to refract light.
• It is not the same as mass density.
• A medium with a higher refractive index is optically denser than one with a lower refractive index.

Refraction by Spherical Lenses

• A lens is a transparent material bound by two spherical surfaces.
• A lens can be either convex (converging) or concave (diverging).
• Convex lenses are thicker in the middle, while concave lenses are thicker at the edges.
• The centres of curvature of a lens are represented by the letter C.
• The principal axis of a lens is an imaginary line passing through the two centres of curvature.
• The optical centre of a lens is the central point through which a ray of light passes without deviation.
• The aperture of a lens is its effective diameter.

This quiz covers the basics of light, reflection, and refraction. It explores how light makes objects visible and its importance in our daily lives.