# Light (प्रकाश) and Its Facts

Physics topic – “Light (प्रकाश) and Its Facts” is important for all competitive exams like CET- Common eligibility Test, SSC CGL, SSC CHSL, RRB NTPC, UPSC and other state civil services exams. In these exams, almost 4-5 questions are coming from Physics. Let’s start the topic- Light (प्रकाश) and Its Facts:-

## Light (प्रकाश) and Its Facts

Light:

Light or visible light is a transverse, electromagnetic wave that can be seen by the human eyes.

Electromagnetic waves: – Electromagnetic waves have not need of any medium to propagate. Light can travel through a vacuum.

So, light is also not required any medium to propagate. It can be travelled in vacuum also.

Important Facts of light:-

• The speed of light in a vacuum is a universal constant in all reference frames.
• The speed of light in a vacuum is fixed at 299,792,458 m/s by the current definition of the meter.
• The speed of light in a medium is always slower the speed of light in a vacuum.
• The speed of light depends upon the medium through which it travels. The speed of anything with mass is always less than the speed of light in a vacuum.

Other Characteristics:-

• The amplitude of a light wave is related to its intensity.
• Intensity is the absolute measure of a light wave’s power density.
• Brightness is the relative intensity as perceived by the average human eye.
• The frequency of a light wave is related to its color.
• The wavelength of a light wave is inversely proportional to its frequency.
• Light ranges in wavelength from 400 nm on the violet end to 700 nm on the red end of the visible spectrum.

Electromagnetic Wave Spectrum: –

Electromagnetic radiation have a wide range of wavelengths, from gamma rays to radio waves measured in meters.

Band of visible light comes, from about 700 nanometers (nm) for red light to about 400 nm for violet light.

Visible light

• White light is the mixture of all 7 color of visible light.
• There are three fundamental (primary) color: BLUE, GREEN, RED.
• Red color of light use in dangerous sign because:  Due to their higher wavelength and less scattering. It can be seen from far distance.

Electromagnetic Wave uses:-

 Category of Radiation Uses Gamma rays Give information on nuclear structure, used to kill the bacteria in marshmallows and to sterilize medical equipment X-rays used to image bone structures Ultraviolet light (UV rays) Preserve food, sterilizing the surgical treatments, finger prints, bees can see into the ultraviolet light. Visible Light It used by humans to see objects Infrared Night vision, Heat sensors, Laser metal cutting Microwave Microwave ovens, Radar, Telecommunication Radio Waves Radio, Television Broadcasts.

Geiger Muller Counter (GM counter): – This device used for the detection and measurement of all types of radiation alpha, beta and gamma radiation

Discovery of electromagnetic waves:

• U.V. waves discovered by: Johann Ritter
• X – Rays discovered by: Wilhelm Roentgen
• Cosmic or radioactive rays discovered by: Antoine Henri Becquerel
• Visible light discovered by: Newton
• Wireless waves discovered by: Guglielmo Marconi.

Scattering of light:

Scattering of light is the phenomenon in which light rays get deviated from its straight path on striking an obstacle like dust or gas molecules, water vapors etc.

Important Fact of scattering of light:

• Violet color light has maximum scattering.
• In foggy weather, yellow color of light use in automobiles because it’s least scattering.
• Sky appears blue due to scattering of light, or due to presence of dust and moisture in atmosphere.
• Low wavelength of light, scatter more.
• At the surface of moon sky appears black because there is no atmosphere, hence scattering of light does not occur.

Light (प्रकाश) and Its Facts

Phenomenon of light: –

1. Absorption
2. Reflection
3. Refraction

1. Light Absorption:

Light absorption is a process by which light is absorbed and converted into energy.

Absorption depends on the electromagnetic frequency of the light and object’s nature of atoms. The absorption of light is therefore directly proportional to the frequency. If they are complementary, light is absorbed.

Important Facts of absorption:

• If an object absorbed all the color of the light, they appear “BLACK”.
• If an object reflect all the color of the light, it appear white.
• When we see any color of any object, it means seen color is reflected color and all other 6 color absorbed.

2. Reflection:

The process through which light rays falling on the surface on an object are sent back or bounces back the light is called reflection of light.

• If an object reflect all the color of the light, it appear white.

3. Refraction:

Refraction is the bending of a wave when it enters a medium where its speed is different.

• The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media.
• The amount of bending depends on the index of refraction.
• “The index of refraction” is defined as the speed of light in vacuum divided by the speed of light in the medium.
• (Index of refraction) N = c/v    where, c is velocity of light in vacuum and v is the velocity of light in medium.

Important Facts of Refraction:

• The depth of swimming pull appears less than its original depth, due to refraction of light.
• A rod emerged in water appears bend due to refraction of light.
• Twinkling of the stars occurs due to refraction of light.
• A glass rod emerged in water, it goes disappear because of refractive index of water and glass are same.
• At the time of sun rise and sun set, sun appears large and oval size due to refraction of light.

Some important points:

Incident ray: The ray of light falling on the surface of a mirror is called incident ray.

Normal: A line perpendicular or at the right angle to the mirror surface at the point of incidence is called normal.

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

Total Internal Reflection:

Total internal reflection occurs when the light rays travel from a more optically denser medium to a less optically denser medium.

Conditions of Total Internal Reflection:

• The light ray moves from a denser medium to less dense medium.
• The angle of incidence must be greater than the critical angle.

FACTS of Total Internal Reflection:

• Diamond shine too much due to total internal reflection of light.
• Mirage form due to total internal reflection of light.
• Working of optical fiber based upon total internal reflection of light, which is widely use in communication and endoscopy surgery.

Light (प्रकाश) and Its Facts

Diffraction of Light:-

Light bends when it passes around an edge or through a slit that is physically the approximate size of, or even smaller than that light’s wavelength. This bending is called diffraction.

Important Facts of diffraction of light:

• Before sun rise and after sun set, light present in sky due to diffraction of light.
• CD appears like rainbow colors due to diffraction reflection of light simultaneously.
• Shop bubble appears rainbow colorist due to diffraction of light.
• If a kerosene oil pore in water, then layer of water look like rainbow colors due to diffraction and reflection of light.

Dispersion of Light: –

When white light is passed through a glass prism it splits into its spectrum of colors (in order violet, indigo, blue, green, yellow, orange and red) and this process of white light splitting into its constituent colors is termed a dispersion.

Important Facts of dispersion:

• Formation of rainbow occur due to dispersion of light.
• Prizm also showing different color by the dispersion of light.
• Maximum dispersion of light in violet color.
• Minimum dispersion of light in yellow color.

Some Other Important points About Light:

1. At the time of eclipse, rectilinear propagation of light takes place.
2. Due to rectilinear propagation of light shadow forms.
3. At the time of formation of rainbow, sun must behind the rainbow.
4. At the time of noon 12, rainbow can’t be seen.
5. At a time maximum 2 rainbow can see.

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