[Physics Class Notes] on Light Energy Pdf for Exam

This article has been prepared by the subject experts at , discussing important concepts related to light energy. This will help students get conceptual clarity about the topic and will help them solve any question related to the topic. 

Download the free PDF notes of the topic from the website or mobile app of or watch the video lecture to understand the topic well.

Table of Content:

• What is Light?

• What is Light Energy?

• Type of Light Energy

• Properties of Light Energy

• Use of Light

• Use of Light Energy

• Use of Light in Physics 

• Key points from the Chapter

• Frequently asked questions

What is Light?

Light is a form of energy. Light plays a very important role in our lives because without it we would have to survive in the dark walking endlessly without any start or an exit. 

We know that the sun is the natural source of light; however, there are other sources that also emit light such as electric bulbs, candles, torches, kerosene lamps, and so on.

We know that the speed of light is 3 x 108 m/s and nothing can travel faster than this speed. However, according to the theory of relativity, objects can travel faster than this speed.

What is Light Energy?

The energy produced by vibrations of electrically charged particles is light energy. The examples of light energy are:

• The energy that we use to cook food in a microwave oven is a form of light energy.

• Army men use radar to get the searchlight beam on the target.

• Radar systems that are used for ATC (air traffic control) use a form of light energy.

What are the types of Light Energy? 

Following are types of Light Energy – 

• Visible Light – They are the form of electromagnetic energy and can be seen only through naked eyes. Naturally, the Sun is the source of Visible light, Whereas, flashlights, bulbs are some of the man-made sources.

• Infrared Light – They are a form of electromagnetic energy which produces heat. Tv remotes, AC remotes are the most commonly used Infrared light. 

• X-rays and Ultraviolet Light – They are lights with short wavelengths. Used by doctors to monitor the inside of a human body.

 Properties of Light Energy 

• Intensity – The rate at which light emits from the source per unit area is the intensity of light, which is measured in Watts.

• Frequency – the number of waves that can pass via a point in space during the time interval.

• Wavelength – It is the distance between the two consecutive crests or troughs of a wave.

• Polarization – when all the waves vibrate in a single plane, then we can say that the light is polarized.

• Phase – A point in the time period during the cyclic

Uses of Light Energy

• Food formation

• Growth of the human body

• Regulation of Physiology

• Sight and vision

• Heat and temperature

• Drying & evaporation

• For speed regulation

• Source of electrical energy

• Chemical analysis by spectroscopy

• Sanitation of earth

• Killing germs viz: microorganisms

• Signaling system

• Sterilization process

Now, let’s discuss these in detail:

 

Uses of Light

Food formation

• The food we obtain from plants is only possible because of light. A light helps plants in undergoing photosynthesis. The light obtained by the plants is used in preparing food for itself and for human beings.

• We know that Vitamin D is beneficial for us. So, for gaining this energy for free, Sun is the best provider of this nutrient. 

• The light energy we get from the sun can be utilized for domestic purposes like we can use these for operating solar cookers. We also find the use of sunlight for solar water heaters.

• Light is the only source that helps us see beautiful things around us.

Light maintains our body clock

As we can see, we remain active all day and relaxed at night. Similarly, sunflowers rejoice by spreading their leaves and they always face towards the sunlight; however, they fade after the sunrise.

 

In Physics

• Visible light in the electromagnetic radiation in the range of an electromagnetic spectrum that is perceived by the human eye.

• A human eye can distinguish between 6,000 shades of a particular color, all this is possible because of the light. Light has vari
  ous electromagnetic spectrum, where each spectrum has seven following colors (VIBGYOR):

1. Violet

2. Indigo

3. Blue

4. Green

5. Yellow

6. Orange 

7. Red

Each of these colors has a unique wavelength and frequency. And when light falls on an object, it absorbs light of some wavelengths and reflects others. 

So, when the light falls on any object, it absorbs some spectra of light and reflects the remaining and whichever color it reflects, becomes the visible color. However, the colors that are absorbed are not visible to us, only the color it reflects is visible to us.

 

In Spectroscopy

We use spectroscopy for both quantitative and qualitative analysis of various compounds; however, all this is possible because of light. Spectroscopy also has many forms and these vary with the wavelength of light used in each.

 

For Sterilization

The ultraviolet component of light is an excellent sterilizing agent. You might’ve noticed the use of UV lamps in supermarkets and hospitals; these UV lamps are the best sterilizers.

 

The UV rays kill the microbes in the air keeping the area of the supermart or hospital sterile. That’s why UV light is considered a dry method of sterilization.

 

Sunlight is one of the effective sterilizing agents that helps kill all the microbes present in the blankets or other mattresses of hospitals.

 

Light as a Source of Drying and Evaporation

Sunlight helps in the drying of wet clothes, the soil, forests, air, and rocky surfaces due to their ability to heat up. Light also helps water evaporate from oceans and ponds. This drying is helpful in the following two ways to live on earth:

 

1. Water Cycle:

The water cycle is a way by which freshwater on the earth’s surface keeps recycling for drinking and irrigation purposes. The water from the surface evaporates to form clouds.

 

These clouds burst in the form of raindrops to return the freshwater to the surface of the earth. So because of the rain, we get fresh water to drink and also use it for irrigation.

Without the water cycle, all the water on the earth would have remained in the ocean we would have no water to drink.

 

2. Scavenging:

Sunlight keeps our nature clean. Sunlight prevents the growth of weeds, algae, and other microbes keeping the environment clean and tidy. 

 

Because of the sunlight, the dead bodies and other waste material get dried and lose their mass, and degrade  (reduced in quantity) fast.

 

Key Points from the Chapter:

• Light is a form of energy

• There are two types of sources: natural and manmade, Sun is the natural source whereas, bulbs, candles, lights are the man-made sources.

• The speed of light is the fastest 

• Light and Energy has various uses and applications

• Without light, the process of  food formation and the very existence of life on the Earth will be impossible