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Class 10 Science: Light Reflection and Refraction Notes

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Class 10 Science: Light Reflection and Refraction Notes

Light – Light is a form of electromagnetic radiation that is visible to the human eye.

 OR

Light is a form of energy that travels in waves that enable us to see.

Properties of Light –

  1. Light is a form of electromagnetic radiation, it does not require any medium.
  2. Light travels in a straight line.
  3. Light has dual nature.
  4. Light casts shadows.
  5. Speed of light in vacuum 3x108 m/s.

When light fall on a surface then –

  1. Reflection
  2. Refraction

Reflection of light – Reflection of light is the phenomenon in which light is bounced back after stricks a polished surface, resulting in a change of direction of the light.

Laws of Reflection –

  1. The angle of incidence is equal to the angle of reflection.
  2. The incident ray, the reflected ray, and the normal to the reflecting surface at the point of incidence, all lie in the same plane.

Image –  An image is a representation of an object or scene that is formed by the reflection, refraction, or diffraction of light.

OR

An image is a representation where at least two light rays actually meet or appear to meet.

Real Image – A real image is an image that is formed when light rays actually meet, and it can be projected onto a screen or a surface.
A real image can be obtained on the screen.
The real image is always inverted.

Virtual Image – A virtual image is an image that appears to be formed where the light rays do not actually meet, and it cannot be projected onto a screen or a surface, it can be seen by the eye directly.
The virtual image is always Erect.

The image on the plane mirror –

Characteristics of Image –

  1. Virtual & Erect
  2. Equal to the object
  3. Behind the same distance as the object
  4. Laterally inverted.

Lateral Inversion – Apparent reversal of the mirror images left & right when compared with the object.

OR

When an image is laterally inverted, the left side of the object appears on the right side of the image and vice versa.

Spherical Mirror

Spherical Mirror – A spherical mirror is a mirror that is curved in such a way that its surface forms a part of a sphere. Spherical mirrors are classified into two types: concave spherical mirrors and convex spherical mirrors.

P – Pole
C – Centre of Curvature
R – Radius
F – Focus
f – focal length
MN – Aperture

Types of Spherical mirrors –

  • Concave Mirror
  • Convex Mirror

Concave Mirror – A concave spherical mirror is a mirror in which the reflecting surface bulges inward.
The centre of the sphere is called the centre of curvature and is located behind the mirror.
A concave mirror is also called a converging mirror because it converges the rays of light that are parallel to its principal axis and diverges the rays that are directed toward its centre of curvature.

Convex Mirror – A convex spherical mirror is a mirror in which the reflecting surface bulges outward.
The centre of the sphere is called the centre of curvature and is located in front of the mirror.
A convex mirror is also called a diverging mirror because it diverges the rays of light that are parallel to its principal axis and converges the rays that are directed toward its centre of curvature.

Rules to making Ray diagram by Concave Mirror –

  1. A ray parallel to the principal axis will pass through the focus after reflection.
  2.  A ray incident through focus, after reflection, will parallel to the principal axis.
  3. A ray passes through the centre of curvature is reflected back on the same path.
  4. A ray incident on the pole is reflected making an equal angle.

Ray diagram of images formed by Concave Mirror –

1. When an object at infinity –

  • Position – at F
  • Nature – Real & Inverted
  • Size – Point Size

2.  When an object is beyond C –

  • Position – between F & C
  • Nature – Real & Inverted
  • Size – Small size

3. when the object at C –

  • Position – at C
  • Nature – Real & Inverted
  • Size – Small size

4. When the object between C & F –

  • Position – at beyond C
  • Nature – Real & Inverted
  • Size – Enlarged

5. When the object at F –

  • Position – at infinity
  • Nature – Real & Inverted
  • Size – Highly Enlarged

6. When an object is between P & F –

 

  • Position – behind the mirror
  • Nature – Virtual & Erect
  • Size – Enlarged

Uses of Concave Mirror –

  1. Solar Furnaces: Concave mirrors are used to concentrate sunlight in solar furnaces to produce high temperatures for industrial processes.

  2. Reflecting Telescopes: Concave mirrors are used as the primary mirrors in reflecting telescopes to collect and focus light from distant objects.

  3. Medical Equipment: Concave mirrors are used in medical equipment, such as ophthalmic instruments, to examine the eye.

  4. Automotive Mirrors: Concave mirrors are used as the reflective surface in some rearview mirrors in automobiles.

  5. Floodlights: Concave mirrors are used to focus light in floodlights, providing a concentrated beam of light for outdoor use.

  6. Searchlights: Concave mirrors are used in searchlights to produce a bright, collimated beam of light.

  7. Headlights: Concave mirrors are used in some vehicle headlights to focus the light and increase its brightness.

  8. Studio Lights: Concave mirrors are used in some studio lights to focus the light and produce a bright, directional beam.

  9. Periscopes: Concave mirrors are used in periscopes to reflect light and form an image that is laterally inverted.

  10. Magnifying Glasses: Concave mirrors are used as the reflecting surface in some magnifying glasses, to magnify the size of an object.

Rules to making Ray diagram by Convex Mirror –

  1.  A ray of light parallel to the principal axis of a convex mirror appears to diverge from the focus point.
  2. A ray directed toward the focus of the convex mirror will parallel the principal axis.
  3. A ray is directed towards to centre of curvature and reflected back on the same path.
  4. A ray incident to a pole reflects with the same angle.

1. When an object at infinity –

  • Position – at F
  • Nature – Virtual & Erect
  • Size – Point

2. When the object between P & infinity –

  • Position – between P & F
  • Nature – Virtual & Erect
  • Size – Small

Uses of Convex Mirror –

  1. Automotive Mirrors: Convex mirrors are used as the reflective surface in some side mirrors on vehicles, as well as in some backup cameras.
  2. Safety Mirrors: Convex mirrors are used in areas where visibility is limited, such as in corners, hallways, and intersections, to provide a wider field of view.
  3. Store Displays: Convex mirrors are used in stores and shopping malls to provide a wider field of view and to enhance security by allowing store employees to see a larger area.
  4. Surveillance Systems: Convex mirrors are used in surveillance systems to cover a wider area and provide a broader view of the surroundings.
  5. Periscopes: Convex mirrors are used in periscopes to reflect light and form an image that is laterally inverted.
  6. Optical Instruments: Convex mirrors are used in optical instruments, such as binoculars, to reflect light and produce an image.
  7. Studio Lights: Convex mirrors are used in some studio lights to spread the light and produce a wide, diffuse beam.
  8. Traffic Control: Convex mirrors are used at intersections and roundabouts to improve visibility and enhance traffic safety.

New Cartesian Sign Convention for spherical mirrors –

  • Object distance (u): Object distance is taken as positive if the object is located to the left of the mirror or lens, and negative if it is located to the right.
  • Image distance (v): Image distance is taken as positive if the image is located to the right of the mirror or lens, and negative if it is located to the left.
  • Height of object and image (h): The height of an object or an image is taken as positive if it is located above the principal axis, and negative if it is located below the principal axis.
  • Focal length (f): The focal length is taken as positive for a convex lens or a concave mirror, and negative for a concave lens or a convex mirror.
  • Magnification (m): Magnification is defined as the ratio of the height of the image to the height of the object, and is taken as positive if the image is erect and magnified, and negative if the image is inverted and reduced.

Mirror Formula –

v = Image distance
u = Object distance
f = Focal length
1/v + 1/u = 1/f

Magnification of Spherical Mirrors – The ratio of the height of an image to the height of the object.
magnification (m) = height of image/ height of the object
m = h’/h
m = -v/u = h’/h

Refraction – Refraction is the bending of light as it passes from one medium to another, such as from air to glass or water.
It occurs because light travels at different speeds in different media, and the speed of light changes as it passes from one medium to another. This change in speed causes the light to bend, and this bending is called refraction.

Cause of Refraction – Light travels at different speeds in different media. The speed of light is higher in a medium with a lower density and lower in a medium with a higher density.
When light passes from one medium to another with a different density, its speed changes and this causes the light to bend.
Speed of light in vacuum = 3x108 m/s.

Some examples of refraction – 

  • Lenses: Refraction is used in lenses to form images. Eyeglasses, camera lenses, and telescopes all use refraction to focus light and form clear images.
  • Mirrors: Refraction occurs when light passes through the curved surface of a mirror and bends, creating an image.
  • Rainbows: Rainbows are formed by the refraction of light in water droplets in the atmosphere. The light enters the droplet at an angle, is refracted, and then exits the droplet at another angle, creating a characteristic spectrum of colors.
  • Optical fibers: Optical fibers use refraction to guide light through a transparent medium, such as glass or plastic. The light enters the fiber at one end and is refracted multiple times as it passes through the fiber, allowing it to travel long distances with little loss of signal strength.
  • Prism: A prism uses refraction to separate light into its component colors, creating a spectrum.
  • Mirages: Refraction can cause light to bend and create the illusion of a pool of water on a hot road, for example.

Refraction through Glass Slab – Refraction through a glass slab occurs when light passes from one medium, such as air, into another medium, such as glass, with a different refractive index. The refractive index of glass is typically higher than that of air, causing the light to bend as it enters the glass.

Lateral Displacement depends on – 

  • Refractive index of glass slab
  • The thickness of the glass slab

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

Snell’s law: Snell’s law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for light passing from one medium to another. This law is expressed mathematically as:
sin(i) / sin(r) = constant

Refractive Index (n) – It is defined as the ratio of the speed of light in a vacuum to the speed of light in a given medium.
n = velocity of light in medium one / velocity of light in medium two
n = V1/V2
n12 = V1/V2
n21 = V2/V1

Absolute Refractive Index – The amount of bending of light in a vacuum, compared to the amount of bending of light in a given medium. It is defined as the ratio of the speed of light in a vacuum to the speed of light in a given medium.
n = c/v
where c = 3x108 m/s
v = another medium
The Refractive Index of a diamond is the highest i.e. 2.42.

Optically Denser Medium – An optically denser medium is a medium that has a higher refractive index than another medium.

Optically Rarer Medium –  An optically rarer medium is a medium that has a lower refractive index than another medium.

Spherical Lens

A spherical lens is a type of lens that has a curved surface that is either convex (bulging outwards) or concave (curving inwards). The curvature of a spherical lens is determined by the shape of a sphere, which is why it is called a spherical lens.

Convex Lens – A convex lens is a type of spherical lens that has a curved surface that bulges outwards.

  • Thin from the corners
  • Thick at centre
  • Converging

Concave Lens –  A concave lens is a type of spherical lens that has a curved surface that curves inward.

  • Thick from the corners
  • Thin at centre
  • Diverging

Rules for image formation by Convex lens –

  1. A ray of light parallel to the principal axis of a convex lens always passes the focus of another side of the lens.
  2. A ray of light passing through the focus will emerge parallel to the principal axis after reflection.
  3. A ray of light passing through the optical centre will pass without any deviation.

Image Formed by Convex Lens –

1. When an object at infinity –

  • Position -at F
  • Nature – Real & Inverted
  • Size – Point sized

2. When an object beyond C –

  • Position – between F2 & C2
  • Nature – Real & Inverted
  • Size – Diminished

3. When object at C1 –

  • Position – at C2
  • Nature – Real & Inverted
  • Size – Small size

4. When the object between F1 & C1

  • Position – beyond C2
  • Nature – Real & Inverted
  • Size – Enlarged

5. When object at F1 –

  • Position – at infinity
  • Nature – Real & Inverted
  • Size – Highly Enlarged

6. When object is between F1 & O (optical centre)

  • Position – Same side of the lens
  • Nature – Real & Inverted
  • Size – Enlarged

Rules for image formation by Concave lens –

  1. A ray of light parallel to the principal axis appears to diverge from the focus located on the same side of the lens.
  2. A ray of light that appears to meet the focus will emerge parallel to the principal axis.
  3. A ray of light passing through the optical centre  will pass without any deviation.

Image Formed by Concave Lens –

1. When an object at infinity –

  • Position – at F1
  • Nature – Virtual & Erect
  • Size – Point sized

2. When the Object between infinity & optical centre –

  • Position – at F1 & O
  • Nature – Virtual & Erect
  • Size – Diminished

Lens Formula –

1/v – 1/u = 1/f

Magnification (m) = The ratio of the height of an image to the height of the object.
magnification (m) = height of image/ height of the object
m = h’/h
m = v/u = h’/h

Power of lens – The power of a lens refers to its ability to change the direction of light and to focus or defocus the light. It is a measure of the refractive power of a lens and is usually expressed in units of diopters (D).
It is the reciprocal of the focal length in meters.
Power (P) = 1/f
S.I. Unit of power  = Diopter (D)

 

 

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