Imagine you're holding a flashlight in a dark room. When you turn it on, you see a bright beam of light. Well, light is a form of energy that makes things visible to our eyes. It's like a messenger that carries information about the world around us.
Light is a form of electromagnetic energy that causes the sensation of vision. Some common phenomena associated with lights are image formation by mirrors, the twinkling of stars, the beautiful colours of a rainbow, bending of light by a medium and many more.
Properties of Light
When light falls on a surface, the following may happen:
The answer lies in the phenomenon of reflection. When light encounters a surface, it bounces back, allowing us to see objects and ourselves. The reflection of light is the phenomenon where light bounces off a surface and changes direction.
(i) Angle of incidence is equal to the angle of reflection.
(ii) The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
Virtual and Real image
Image is a point where at least two light rays actually meet or appear to meet.
Characteristics of Image Formed by Plane Mirror
(i) Virtual and erect.
(ii) The size of the image is equal to the size of the object.
(iii) The image is formed as far behind the mirror as the object is in front of it.
(iv) Laterally inverted.
Lateral Inversion: The right side of the object appears on the left side of the image and vice-versa.
Application of lateral inversion
Properties of Concave mirror
Properties of Convex mirror
Common terms for Spherical mirrors
(i) A ray parallel to the principal axis, after reflection, will pass through the principal focus in the case of a concave mirror or appear to diverge from the principal focus in the case of a convex mirror.
(ii) A ray passing through the principal focus of a concave mirror or a ray that is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis.
(iii) A ray passing through the center of curvature of a concave mirror or directed in the direction of the center of curvature of a convex mirror, after reflection, is reflected back along the same path.
(iv) 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. The incident and reflected rays follow the laws of reflection at the point of incidence (point P), making equal angles with the principal axis.
(i) When object is at infinity
Image Position − At ‘F’
Nature of image – Real, inverted
Size – Point-sized or highly diminished
(ii) When object is beyond ‘C’
Image Position – Between ‘F’ and ‘C’
Nature of image – Real, inverted
Size – Diminished
(iii) When object is at ‘C’
Image Position – At ‘C’
Nature of image – Real, inverted
Size – Same size as that of object
(iv) When object is placed between ‘F’ and ‘C’
Image Position – Beyond ‘C’
Nature of image– Real, inverted
Size – Enlarged
(v) When object is placed at ‘F’
Image Position – At Infinity
Nature of image – Real, inverted
Size – Highly enlarged
(vi) When object is between ‘P’ and ‘F’
Image Position – Behind the mirror
Nature of image – Virtual, erect
Size – Enlarged
(i) When object is placed at infinity
Image Position − At ‘F’
Nature of image – Virtual, erect
Size – Point sized
(ii) When object is placed between pole and infinity
Image Position – Between ‘P’ and ‘F’
Nature of image– Virtual, erect
Size – Diminished
A full length image of a tall building/tree can be seen in a small convex mirror.
Uses of Convex Mirror
1/v + 1/u = 1/f
where, v = Image distance
u = Object distance
f = Focal length
Magnification of Spherical Mirrors
It is the ratio of the height of the image to the height of the object.
m = Height of image/Height of object
⇒ m = hi/ho
Also, m = -v/u
The magnification of the plane mirror is always + 1.
‘+’ sign indicates virtual image.
‘1’ indicates that the image is equal to the object’s size.
The bending of a ray of light as it passes from one medium to another is called refraction.
It is due to a change in the velocity of light while traveling from one medium to another.
Laws of refraction state that:
According to Snell's law, μ1 sin i = μ2 sin r
(i) If light passes from rarer to the denser medium:
μ1 = μR and μ2 = μD
so that,
⇒ ∠i > ∠r
In passing from rarer to a denser medium, the ray bends towards the normal.
(ii) If light passes from denser to rarer medium μ1= μD and μ2 = μR
⇒ ∠ i < ∠r
In passing from denser to rarer medium, the ray bends away from the normal.
The refractive index depends on the nature and density of the medium and color of light. The refractive index is maximum for violet and minimum for red light.
Refractive index, also called the index of refraction describes how fast light travels through the material.
The Refractive Index is dimensionless. For a given material, the refractive index is the ratio between the speed of light in a vacuum (c) and the speed of light in the medium (v). If the refractive index for a medium is represented by n, then it is given by the following formula:
For Convergent or Convex Lens:
Object | Image | Magnification |
∞ | F | m << – 1 |
∞ – 2F | F – 2F | m < –1 |
2F | 2F | m = –1 |
F – 2F | ∞ – 2F | m > –1 |
F | ∞ | m >> –1 |
F – O | In front of lens | m > + 1 |
Given | Real | Virtual |
u | - | - |
v | + | - |
h1 | + | + |
h2 | - | + |
m | - | + |
Image is virtual, diminished, erect, towards the object, m = +ve
The focal length of a lens can be found by the following formula:
Where - f = Focal length of lens.
v = Distance of image from pole
u = Distance of object from pole.
(i) Put the correct signs of known variables according to the sign conventions.
(ii) Do not put the sign of an unknown variable. The sign will automatically show up during calculations.
(iii) If the calculated sign of a variable turns out positive, then the variable calculated is on the other side of the lens, i.e., on the opposite side to the object. However, if the calculated variable is of a negative sign, then it is on the same side as the object.
Two thin lenses are placed in contact with each other
Power of combination:
P = P1 + P2
1/F = 1/f1 + 1/f2
Use sign convention when solving numericals.
Two thin lenses are placed at a small distance a.
1/F = 1/f1 + 1/f2 - a/f1f2
P = P1 + P2 - a P1P2
Use sign convention when solving numerical.
The power of a lens is defined as the reciprocal of the focal length of the lens. Focal length should always be measured in meters.
The unit of power of the lens is 1/meter, which is called a dioptre.
Magnification is defined as the ratio of the height of the image and the height of the object represented by M.
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1. What is the difference between a convex lens and a concave lens? |
2. What is the significance of the refractive index in optics? |
3. How can we derive the lens formula for convex and concave lenses? |
4. What are the laws of refraction, and how do they apply? |
5. How do you calculate the power of a lens, and what does it indicate? |
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