At dawn and dusk, large amount of blue and violet light(shorter wavelength) has been scattered so, the light that is recieved by an observer is mostly of a longer wavelength and therefore appears to be red.

A lens is a piece of transparent glass bounded bytwo refracting surfaces which are usually spherical.

Two uses of the lens are:

1. It is used in the human spectacle.
2. It is used in scientific projects.
   

The different types of lens are:

Convex lens: A lens that is thick in the middleand thin at the edge is called aconvex lens.

Concave lens: A lens that is thick at the edgesbut thin in the middle is called aconcave lens.

a. Principal axis:
The line joining both the centers of curvatureof a lens is called the principal axis.

b.Optical center:
The geometrical center of a lens is known asits optical center.

c. Center of curvature:
The center of the sphere from whichthe lens has been cut is called the center of curvature.

d. Focal length:
The distance between principle focus andoptical center is called the focal length.

e. Far point:
The distance point from the eyeseen clearly is called the far point.

f. Near point:
The...

Theprocess of adjusting thedistancebetween a lensand the screen so that the image becomes clear anddistinct is called focusing.

Focusing is done to produce a sharp and clearimage inthe camera, microscope, telescope, etc.

a. at 2F

Nature: Real and inverted
Position: 2F

b. Between 2F and F

Nature: Real and inverted
Position: Beyond 2F

c. Af F

Nature: Real and inverted
Position: At infinity

d. Between F and O

Nature: Virtual and erect
Position: Beyond the object

e. Beyond 2F

Nature: Real and inverted
Position: Between F and 2F

When the object is placed between F and O of a convex lens, the lens forms an erect and enlarged image.

Nature: Virtual and erect
Position: Between F and O

Thepower of the lens is defined as the reciprocal of its focallength in meter.
Its SI unite dioptre (D).

The ratio of the height of the image to the heightof the object is called magnification.

As we know that, theinstrument that is used to produce animage of anobject with the help of a lens, is called an optical instrument.In the same way, the camera works on the basis ofrefraction or reflection of light. So, it is called an opticalinstrument.

a. Pupil
It controls the amount of light entering into the eye.

b. Iris
It is responsible for controlling the diameter and size of the pupil.

c. Cornea
It protects the eye and allows the light to pass.

d. Eye lens
It focuses the image changing the focal length.

e. Ciliary muscle
It helps in changing the focal length of the eye lens.

f. Retina
It sends the electric impulses to the brain through the optic nerve.

g. Optic nerve
It acts as the medium to transmit the impulses to and from the brain and retina.

...

The distant point from the eye which can be seen clearly is called the far point of the eye. The normal range of vision of the human eye is about 25 cm.

Myopia is the defect of vision in which a person can see a nearby object but cannot see distant objects.

Its causes are:

1. Due to the shorting of focal length high converging power of the eye lens.
2. Due to the elongation of eyeballs.

Its remedy is:

1. It can be corrected by using a concave lens of suitable focal length.

Hypermetropia is a defect of vision in which a person can see distant objects clearly but cannot see nearby objects.

Its causes are:

1. Due to the increase of focal length low converging power of the eye lens.
2. Due to the contraction of the eyeball.

Its remedy is:

1. It can be corrected by using a convex lens of a suitable focal lenght.

As we know that, a lens that is thick in the middle and thin as the edges is a convex lens. It converges the parallel beam of light at a point after refraction. So, it is called a converging lens.

As we know that a lens that is thin in the middle and thick at the edges is called a concave lens. It diverges the parallel beam of light to different points after refraction. It has a virtual focus. So, it is called a diverging lens.

As we know, myopia is the defect of vision in which a person can see nearby objects but cannot see distant objects. While using a concave lens, the rays of the light coming from a nearby object are first diverged by the concave lens and by the eye lens. Due to the combined diverging action of both lenses, the image is formed at the retina. So, a concave lens is used in spectacles to remove myopia.

a. Convex and concave lens.

b. Virtual image and real image.