Students must go through these JAC Class 10 Science Notes Chapter 10 Light Reflection and Refraction to get a clear insight into all the important concepts.
JAC Board Class 10 Science Notes Chapter 10 Light Reflection and Refraction
→ Light: Light is an electromagnetic radiation, which produces the sensation of sight in our eyes.
- Light is a form of energy that travels in the form of waves.
- These waves do not require a material medium for their propagation. They are non-mechanical waves.
- These waves travel at a speed of 3 x 108 m s-1 in vacuum.
- The wavelength of visible light ranges from 4 x 10-7m to 8 x 10-7m.
→ Image: When a number of rays starting from a point on an object, after reflection or refraction, meet at another point or appear to meet (i.e., give illusion of meeting at another point), then the second point is called the image of the first point. A group of such rays of light is called a beam of light.
Ray of light: A straight-line path joining one point to another in the direction of propagation of light is known as a ray of light.
- The image formed is called a real image, if the rays actually meet after reflection or refraction. It can be obtained on a screen.
- If the rays do not meet actually at a point, but appear to meet, after reflection or refraction, then the image is called a virtual image. It cannot be obtained on a screen.
- When a parallel beam of light is incident on a shining plane or smooth surface, the beam remains parallel after reflection in a specific direction. Such a reflection is called regular reflection.
- Example: the reflection of light by a plane mirror.
- In case of irregular reflection, the reflected beam of light does not remain parallel in specific direction but spreads over a wide region.
- Example : reflection from a book, a table, a chair, etc.
→ Laws of reflection:
- The angle of incidence is equal to the angle of reflection, i.e., i = r.
- The incident ray, the normal to the mirror at a point of incidence and the reflected ray, all lie in the same plane.
→ Image formation by a plane mirror:
- The image formed by a plane mirror is always virtual and erect.
- The image formed is as far behind the mirror, as the object is in front of it.
- The size of the image is equal to that of the object.
- The image formed is laterally inverted, i.e., the left side of the object seems to be the right side of the image and vice versa.
(The phenomenon, by which the left side of the object becomes the right side of the image and right side of the object becomes the left side of the image, is called lateral inversion.)
→ Spherical mirrors: A mirror whose reflecting surface is a part of a hollow sphere is called a spherical mirror. The reflecting surface of a spherical mirror is curved inwards or outwards.
The curved mirrors are of two types:
- Concave mirror and
- Convex mirror.
→ Concave mirror: A spherical mirror with reflecting surface curved inwards is called a concave mirror. Concave mirror forms either a real or virtual image of the object depending on the object distance.
→ Convex mirror: A spherical mirror with reflecting surface curved outwards is called a convex mirror. Convex mirror forms a virtual image of the object for all object distances.
→ Terminology used in respect of spherical mirrors:
(1) Pole (P): The centre of the reflecting surface of a sperical miror is a point called the pole (P) of the mirror.
(2) Centre of curvature (C) : The centre of curvature of a spherical mirror is the centre of the hollow sphere, of which the reflecting surface of the spherical mirror forms a part.
(3) Radius of curvature (R): The radius of curvature of a spherical mirror is the radius of the hollow sphere, of which the reflecting surface of the spherical mirror forms a part.
(4) Aperture : The diameter of the edge of the reflecting surface of a spherical mirror is called the aperture of the mirror.
(5) Principal axis : The imaginary straight¬line passing through the pole (P) and the centre of curvature (C) of a spherical mirror is called the principal axis of the mirror.
(6) Principal focus (F): The point on the principal axis of a concave mirror, at which rays of light incident on the mirror in the direction parallel to the principal axis (actually) meet / intersect after reflection from the mirror is called the principal focus (F) of the concave mirror. When rays parallel to the principal axis are incident on a convex mirror, the reflected rays appear to come from a point on the principal axis. This point is called the principal focus of the convex mirror.
(7) Focal length (f): The distance between the pole (P) and the principal focus (F) of a spherical mirror is called the focal length (f).
→ Selection of rays for locating the image formed by a spherical mirror:
- A ray parallel to the principal axis, after reflection, will pass through the principal focus (F) in case of a concave mirror or appear to diverge from the principal focus (F) in case of a convex mirror.
- A ray passing through the principal focus (F) of a concave mirror or a ray which is 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 a point P (pole of the mirror), on the concave mirror or a convex mirror is reflected obliquely, following the laws of reflection.
Out of these four types of rays, only two types of rays are necessary to locate the position of an image of a point object.
→ Image formation by a Concave mirror:
| Position of the object | Position of the image | Size of the image | Nature of the image |
| At infinity | At the focus F | Highly diminished, point-sized | Real and inverted |
| Beyond C | Between F and C | Diminished | Real and inverted |
| At C | At C | Same size | Real and inverted |
| Between C and F | Beyond C | Enlarged | Real and inverted |
| At F | At infinity | Highly enlarged | Real and inverted |
| Between P and F | Behind the mirror | Enlarged | Virtual and erect |
→ Image formation by a Convex mirror:
| Position of the object | Position of the image | Size of the image | Nature of the image |
| At infinity | At the focus F, behind the mirror | Highly diminished, point-sized | Virtual and erect |
| Between infinity and the pole P of the mirror | Between P and F, behind the mirror | Diminished | Virtual and erect |
→ New Cartesian sign convention for reflection by spherical mirrors:
- The object is always placed to the left of the mirror. This implies that the light from the object falls on the mirror from the left-hand side.
- 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.
- The distances measured perpendicular to and above the principal axis of the mirror (along + Y-axis), are taken as positive.
- The distances measured perpendicular to and below the principal axis (along -Y-axis), are taken as negative.
→ Magnification prpduced by a spherical mirror: The ratio of the height of the image to the height of the object is called the magnification (m). It is the relative extent to which the image of the object is magnified with respect to the size of the object.
→ Refraction: When a ray of light travels obliquely from one transparent medium to another, its speed changes. Therefore, at the boundary separating the two media, there occurs a change in its direction of propagation. This phenomenon is called refraction of light.
→ Laws of refraction:
(1) 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.
(2) The ratio of the 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 known as Snell’s law of refraction. (This is true for angle 0° < i° < 90°)
If i is the angle of incidence and r is the angle of refraction,
\(\frac { sin i }{ sin r }\) = constant = n21
where, n21 is known as the refractive index of medium 2 with respect to medium 1.
The above equation is the mathematical representation of the Snell’s law.
→ Absolute Refractive Index: The ratio of the speed of light in vacuum (c) to the speed of light in medium (v), is called the absolute
refractive index of the medium.
nm = \(\frac { c }{ v }\)
Absolute refractive index of any medium is always more than 1.
→ Relative Refractive Index: The ratio of the speed of light v1 in medium 1, to the speed of light v1 in medium 2, is called the relative refractive index of medium 2 with respect to medium 1 and is represented by the symbol n21. v1 and v2 correspond to the same frequency of light.
n21 = \(\frac{v_1}{v_2}\)
Also, n21 = \(\frac{n_2}{n_1}\) where, n2 = \(\frac{c}{v_2}\) and n1 = \(\frac{c}{v_1}\)
Here, v1 is the speed of light in medium 1, v2 is the speed of light in medium 2, n1 is the absolute refractive index of medium 1 and n2 is the absolute refractive index of medium 2, for the given frequency (given colour) of light.
→ Lateral shift: When a ray of light is refracted at two parallel refracting surfaces, the ray is shifted sideward. This sideward displacement of a ray of light is called lateral shift.
The amount of lateral shift depends upon the perpendicular distance between two parallel refracting surfaces as well as upon the angle of incidence and the refractive index of the second medium with respect to the first medium.
→ Terminology used in respect of lens:
(1) Centre of curvature (C): The centre of a transparent (glass) sphere, of which the curved surface of a lens forms a part is called the centre of curvature C of the respective spherical surface.
Lens has two centres of curvatures C1 and C2.
(2) Principal axis : The imaginary straight-line passing through the two centres of curvature C1 and C2 of a lens, is called the principal axis of the lens.
(3) Radius of curvature (R): The radius of a transparent (glass) sphere of which the curved surface of a lens forms a part is called the radius of curvature R of the respective spherical surface of the lens. Lens has two radii of curvature R1 and R2.
(4) Optical centre (O): The central point of a lens on the principal axis of the lens is called an optical centre O of the lens.
(5) Principal focus (F): When the rays parallel to the principal axis of a convex lens are refracted through the lens, they converge at a point on the principal axis. This point is called the principal focus F of the convex lens.
When the rays parallel to the principal axis of a concave lens are refracted through the lens, they appear to diverge from a point on the principal axis. This point is called the principal focus of the concave lens. A lens has two principal foci F1 and F2 on either side of the lens.
(6) Focal length (f): The distance of the principal focus from the optical centre of a lens is called the focal length f of the lens.
(7) Aperture : The effective diameter of the circular outline of a spherical lens is called the aperture of the lens.
→ Selection of rays for image formation by lens:
- A ray of light from the object, parallel to the principal axis, after refraction through a convex lens, passes through the principal focus on the other side of the lens.
- A ray of light from the object, parallel to the principal axis, after refraction through a concave lens, appears to diverge from the principal focus located on the same side of the lens.
- A ray of light passing through the principal focus, after refraction through a convex lens, will emerge parallel to the principal axis. A ray of light directed towards the principal focus on the other side of a concave lens, after refraction, will emerge parallel to the principal axis.
- A ray of light passing through the optical centre of a lens will emerge without any deviation. (This is true for a convex lens as well as a concave lens.)
From the above three types of rays any two types of rays will locate the position of the image.
→ Image formation by a Convex Lens:
| Position of the object | Position of the image | Relative size of the image | Nature of the image |
| At infinity | At focus F2 | Highly diminished, point-sized | Real and inverted |
| Beyond 2F1 | Between F2 and 2F2 | Diminished | Real and inverted |
| At 2F1 | At 2F2 | Same size | Real and inverted |
| Between F1 and 2F1 | Beyond 2F2 | Enlarged | Real and inverted |
| At focus F1 | At infinity | Infinitely large or Highly enlarged | Real and inverted |
| Between focus F1 and optical centre O | On the same side of the lens as the object | Enlarged | Virtual and erect |
→ Image formation by a Concave Lens:
| Position of the object | Position of the image | Relative size of the image | Nature of the image |
| At infinity | At focus F1 | Highly diminished, point-sized | Virtual and erect |
| Between infinity and optical centre O of the lens | Between focus F1 and optical centre O | Diminished | Virtual and erect |
→ Sign convention for spherical lens : It is similar to that followed for spherical mirrors, but the optical centre of a lens is chosen as the origin of the co-ordinate system.
→ Magnification produced by a lens: The magnification (m) produced by a lens is defined as the ratio of the height of the image to the height of the object.
→ Power of a lens : The reciprocal of the focal length of a lens is called the power of the lens (P).
P = \(\frac { 1 }{ f }\)
SI unit: The dioptre (D)
1 D = 1 m-1
The power of a convex lens is positive and that of a concave lens is negative.