Wave -

Light

Light is the formation of energy which causes the sensation of sight. It travels in waves and can move through a vacuum at a speed of about 300,000 kilometers per second.

Refraction of light

Refraction is the bending of light as it passes from one medium to another with a different density. For example, when you place a straw in a glass of water, it appears bent or broken at the water’s surface due to refraction.
This happens because the speed of light changes as it moves between different materials.

When light enters a rarer medium (like from water to air), it speeds up and bends away from the normal.

When light enters a denser medium (like from air to water), it slows down and bends toward the normal.

Rarer and Denser Medium

Denser Medium: A medium in which the speed of light is slower. This typically happens when light moves through a material that has more optical density. Examples include glass, water, or oil.

Rarer Medium: A medium in which the speed of light is faster. This is usually a material that has less optical density, such as air or vacuum.

Refraction of light through glass slab

Incident ray
The ray of light which strikes a transparent medium is called incident ray.

Refracted ray
The ray of light that passes in the second medium after refraction is called refracted ray.

Angle of incidence
The angle made by incident ray to the normal is called angle of incidence.

Normal
The perpendicular drawn at the point on incidence is called normal.

Angle of refraction
The angle made by refracted ray to the normal is called angle of refraction.

Emergent ray
The ray of light which emerges out of the second medium is called emergent ray.

Emergent angle
The angle made by emergent ray to the normal is called emergent angle

Lateral displacement
The perpendicular distance between emergent ray and incident ray is called lateral displacement or lateral shift.

Law of Refraction of light

a. When light passes from one optical medium to another medium, the incident ray, the normal and the reflected ray all lie on the same plane at the point of incidence.
b. The ratio of sine of the angle of incidence to the sine of the angle of refraction remains constant for a pair of media. The constant value is refractive index of pair of media which is denoted by µ.

Refractive index can also be calculated by given formula.

The refractive index can be defined as the ratio of sine of angle of incidence to the sine of angle of refraction for a given pair of media.

Real depth and Apparent depth

Real depth can be defined as the depth at which an object is placed in a beaker or liquid such that the observer and object are in the same medium.
Apparent depth can be defined as the depth of the object measured by the observer when the object and observer are in different media.

Critical angle

Critical angle for a certain medium can be defined as the angle of incidence in a denser medium when the corresponding angle refraction in rarer medium becomes 90°. For example, the value of critical angle for glass medium is 42°, for water medium is 49° and for diamond is 24°.

Total internal reflection of light

The phenomenon in which light arriving at the boundary from one medium to another are not refracted into the second medium, but completely reflected back into the first medium is called total internal reflection of light.
Example: Diamonds are often cut in such a way that when a beam of light penetrates them, it is subjected to entire internal reflections on multiple faces. When the angle of incidence at any face is less than 24°, light shines through, making the diamond look brilliant.

Consequences of total internal reflection of light

a. Sparkling of diamond: Because diamond has a high refractive index, its critical angle is small (24°). This makes it easy for light hitting the diamond-air boundary to have an incidence angle greater than the critical angle, causing total internal reflection within the diamond.

What is the main cause of sparkling of a diamond?

Ans: Diamonds are cut with many angled faces so that the maximum amount of light entering the diamond reflects internally multiple times. This causes total internal reflection, which makes the diamond sparkle.

b. Shining of a surface: When light moves from water (a denser medium) to air (a rarer medium), the angle of incidence at the water-air boundary can exceed the critical angle, causing total internal reflection. This reflection happens on the surface of an air bubble inside the water, making it appear shiny when observed from outside.

Fig: An air bubble inside the water

c. Mirage: A mirage is caused by total internal reflection of light due to temperature differences in air layers near the ground. On a hot day, the ground heats the air just above it, creating a gradient where the air close to the surface is much warmer and less dense than the cooler, denser air above it. As light from the sky travels through these layers, it bends, or refracts, due to the gradual change in air density. When the angle of incidence becomes larger than the critical angle between layers, total internal reflection occurs, causing the light to bend back upwards toward the observer’s eyes. This reflected light appears to come from the ground, creating the illusion of water, as it looks like a reflection of the sky or distant objects.

Fig: Total internal reflections during the formation of mirage

Application of total internal reflection of light

Optical fibre

The thin optical medium used for the transmission of light in fibre optics is called optical fibre. The inner part of an optical cable has a core which is surrounded by a cladding which is made of material of a lower refractive than the core creating a suitable environment for total internal reflection in the core.

Fig: Total internal reflection in optical fibre

a. Use of optical fibres in telecommunication

In communication technology, optical fibres are used for the rapid transmission of signals or data in the form of light waves using the principle of total internal reflection.

b. Use of optical fibres in the medical field

Endoscopy and colonoscopy

Endoscopy is the non surgical method for examining the internal organs with an optical fibre and camera. For example, endoscopy can be used to diagnose ulcers in the stomach.
Colonoscopy is a type of endoscope that is used to examine the colon and large intestine by inserting it through the rectum

Keyhole surgery

It is a procedure performed by a surgeon to operate inside the body through a small incision in the skin. Laparoscope is used in this procedure which incorporates bindles of optical fibres and a camera to send light inside the body and capture the shape of the internal organs, which is displayed on a monitor.

Dispersion of light

Dispersion of light is the process by which white light separates into its different colors when it passes through a medium, like a glass prism. Each color bends by a different amount due to varying wavelengths, creating a spectrum of colors (violet, indigo, blue, green, yellow, orange, and red).

Causes of dispersion of light

Dispersion of light occurs when white light passes through a prism, creating a spectrum of seven colors and illustrating that white light is a combination of these distinct colors. The prism separates the colors through refraction, which happens as light strikes the glass. Variations in wavelength lead to different velocities in the medium, causing each color to deviate by a unique angle. Consequently, red light, with the longest wavelength, deviates the least and appears at the top of the spectrum, while violet light, with the shortest wavelength, deviates the most and is found at the bottom.

Rainbow

A rainbow is an arch of colors visible in the sky, formed by the refraction and dispersion of sunlight as it passes through rain or other water droplets in the atmosphere. A rainbow forms when sunlight enters a water droplet, slowing down and bending as it moves from air to denser water. The light reflects off the droplet’s inside, separating into its component colors. When the light exits the droplet, it creates a rainbow.

Lens

A lens is a transparent medium that has at least one spherical surface. Mainly there are two types of lenses. They are convex lens and concave lens.

The lens which is thicker at the middle part than that at the edges is a convex lens. It is known as a conversing lens because of its ability to converge light rays. It is used in devices such as spectacles, cameras, microscopes, projectors, etc. The lens which is thinner in the middle part than that at the edges is called concave lens. It is known as a diverging lens because of its ability to diverge light rays. It is used as serving spy holes in door, laser devices, etc.

Terminologies related with lens

a. Centre of curvature (C): Centre of curvature is the center of the sphere that forms the curved part of the lens.
b. Radius of curvature (R): The radius of curvature is the distance from the center of the curvature to the surface of the lens. It is denoted by R in a ray diagram.
c. Optical center (O): It is the geometrical center of the lens, where the principal axis intersects the lens surface. It is denoted by O in a ray diagram.
d. Principal axis (P.A): The line passing through the optical center and the two centers of curvature of the lens is called principal axis.
e. Principal focus (F): The point on the principal axis where the beam of light rays parallel to the principal axis converge or from where the beam of light appears to diverge after the refraction through the lens is called the focus.
f. Focal length (f): The distance between the principal focus (F) and the optical center (O) of the lens is called its focal length.
g. Power of a lens: The capacity of a lens to converge or diverge the light rays falling on it is called its power.

Magnification: The magnification produced by a lens is defined das the ratio of the size (or height) of the image to the size (or height) of the object.

Image formed by the convex lens

Image formed by concave lens

When object is placed between infinity and optical centre

The image is
i. virtual
ii. erect and
iii. diminished

Human eye

The optical instrument that forms the images of objects naturally by refracting the light through a convex lens is the human eye.

Cornea: The transparent front layer that refracts light entering the eye.
Pupil: The opening in the center of the iris that controls the amount of light entering the eye.
Iris: The colored part of the eye that regulates the size of the pupil and controls light exposure.
Lens: A transparent structure that further focuses light onto the retina; it adjusts shape for near or distant vision (accommodation).
Ciliary muscles: These are the flexible muscle attached to the lens which play a crucial role in the process of accommodation.
Retina: The innermost layer containing photoreceptor cells (rods and cones) that convert light into electrical signals.
Optic Nerve: Transmits visual information from the retina to the brain for processing.
Vitreous Humor: A gel-like substance filling the eye’s interior, maintaining its shape and allowing light to pass through.
Sclera: The white, outer layer of the eye that provides structure and protection.

Accommodation of eye

Accommodation is the process by which the eye adjusts its focus to see objects clearly at various distances. When viewing nearby objects, the ciliary muscles contract, allowing the lens to become thicker and more curved, increasing its optical power to bend light more sharply for near vision. Conversely, when looking at distant objects, the ciliary muscles relax, increasing tension on the zonules, which flattens the lens and reduces its optical power for distance vision.

Far point and near point of human eye

The far point is the farthest distance from the eye at which it can see objects clearly. For a normal eye, this point is at infinity.
The nearest distance from the eye at which it can see objects clearly is called the near point of the eye. For normal eye, the near point is at a distance of 25 cm from the eye. For a person with no visual defect, the range of human vision is from 25cm to infinity (∞)

Defects of vision

A defect of vision refers to any condition where the eye is unable to focus light correctly onto the retina, resulting in blurred or distorted vision. There are two types of defects of vision in human eyes. They are shortsightedness (myopia) and longsightedness (hypermetropia).

Shortsightedness or myopia

Shortsightedness is that defect of vision in which the person/eye can see the nearby objects but cannot see the far/distant objects clearly/distinctly.

Causes of shortsightedness

Elongation of the eye ball or thickening of the eye lens or high conversing power of eye lens.
Shortening/decreasing of the focal length of the eye lens than required due to inability of ciliary muscles to contract enough.

Correction of shortsightedness

Shortsightedness can be corrected by using spectacles with concave lens of suitable power. The concave lens so used diverges the parallel rays of light rays coming from the distant object before entering the eyes so that after refracting, these distant rays through the cornea, they are finally converged/ focused on the retina by the eye lens to form the image exactly on the retina and thus the object is seen clearly.

Long sightedness/ hypermetropia

Longsightedness is the defect of vision in which the person/eye can see the distant/far objects clearly but cannot see the nearby objects clearly.

Causes of longsightedness

Shortening of the eye ball or thinning of the eye lens.
Increasing of the focal length of the eye lens or the low converging power of the eye lens.

Correction of longsightedness

The longsightedness is corrected by using the spectacles with the convex lens of suitable power. The convex lens converges the light rays from the nearby object which are further converged by the eye lens on the retina the image is exactly formed on the retina and such object is clearly seen.

Other ways for correction of defect of vision apart from using spectacles

a. Contact lens

A contact lens is a thin, curved lens placed directly on the surface of the eye to correct vision. Contact lens require more careful handling than spectacles. They are to be cleaned and disinfected before they are reused. To prevent infection, one should always wash his/her hands before wearing contact lenses and before removing them.

b. Laser eye surgery

The process of correcting defects of vision by reshaping the cornea using special type of ultraviolet laser i.e. excimer laser is called laser eye surgery. LASIK (Laser assisted in situ keratomileusis) is the most popular technique among different laser eye surgery techniques. In this, the laser beam focused on the cornea cuts out a flap. The cutout flap is flipped to reshape the cornea.

Effects of injuries to the cornea in the eyes

The refractive index of cornea is 1.376 and its converging power is about +43D. Bacterial, viral and fungal infections in the cornea can cause corneal diseases and vision problems. The corneal infection causes corneal ulcer i.e. keratitis.
Corneal edema caused by fluid accumulation between the corneal layers also causes blurred vision.
Keratoconus occurs when the surface of the cornea changes into a conical shape, causes shortsightedness at first and later the eye loses its vision.

Corneal transplantation

Corneal transplantation is a surgical procedure where a damaged or diseased cornea is replaced with a healthy cornea from a donor. Some people donate their eyes to the eye bank so that other people can use the cornea of their eye after their death.
In our country, Nepal Eye Bank has been established under Tilganga Eye Institute for the collection, storage and distribution of corneas.