Physics 10th - Question Answers
NATURE OF LIGHT AND ELECTROMAGNETIC SPECTRUM
CHAPTER # 15
Q.1: What is Newton’s corpuscular theory of light?
Ans:
This theory which was proposed by Newton is given briefly as under:
- Light consists of tiny (minute) particles called corpuscles.
- These corpuscles are emitted from a source of light (like the sun).
- These corpuscles are traveled in a straight line.
- When these particles enter the eyes, they create the sensation of sight.
- Velocity of light in a denser medium is greater.
Q.2: What is Huygens’s waves theory of light?
Ans:
Huygens proposed a theory known as the wave theory of light. According to this theory:
- Light is a form of energy, and it traveled in the form of waves.
- Medium was essential for the propagation of waves; therefore, it was assumed that all space was filled with a hypothetical medium called ether.
- Waves in different media propagate with different speeds.
- Waves of different wavelengths bend differently.
- The waves of light were so tiny that there had no visible distortion in typical mirrors and lenses and in the formation of shadows.
- Wave theory explained the rectilinear propagation of light, formation of shadows, reflection, refraction, and a few other properties of light.
Q.3: What is quantum theory and dual nature of light?
Ans:
In 1905, Max Planck concluded after an experiment that radiation was emitted in the form of small packets of energy.
- When the light falls on a metal surface, electrons are emitted from it.
- The physical nature of light was not that of a wave but of little packets of light energy called photons.
- Light has a dual nature. Sometimes it behaves as a particle and sometimes as a wave. It never exhibits both characteristics at the same time.
- It explains the existing phenomenon of light through empty space.
Q.4: Write down the difference between corpuscular and wave theory of light?
| Corpuscular Theory | Wave Theory |
|---|---|
| According to this theory light consists of small particles called corpuscles. | According to wave theory, light is a form of energy because it travels in the form of waves. |
| Light particles travel with the velocity of light in a straight line. | Waves travel with the velocity of light but not in a straight line. |
| Light travels with high velocity in a denser medium. | Light travels with slow velocity in a denser medium. |
Q.5: What do you understand by dispersion of light?
Ans:
When a beam of sunlight (white light) falls on a prism, it splits up into seven colors. This phenomenon is called the dispersion of light.
Q.6: Describe an experiment to show that light consists of seven colors?
Ans:
A beam of sunlight is allowed to enter a dark room through a hole. A prism is placed in the path of the beam of light. A band of various colors is produced on the wall. This phenomenon is called the "Dispersion of light."
We know that the amount of refraction of waves depends on their frequencies. This experiment shows that sunlight consists of waves of different frequencies. When these waves pass through the prism, the waves of higher frequency bend more than those of lower frequencies. Due to this, seven colors are obtained which are violet, indigo, blue, green, yellow, orange, and red. We can remember the names and their colors by the word (VIBGYOR).
- The shortest visible wavelength is violet, and its deviation is the greatest.
- The longest visible wavelength is red, and its deviation is the least.
Q.7: What is a rainbow? How does a rainbow form?
Ans:
Rainbow:
The rainbow is an arc of spectral colors formed across the sky during or after rainfall in the morning and evening.
Formation of Rainbow:
When rain falls, drops of water behave like prisms and white light entering the drops of water is split up into colors on refraction. The light enters the raindrop from one side and does not pass through. It is reflected from the opposite side and re-emerges through the side it entered but at an angle of about 42° to its original direction. We see an arc with red on the top and violet below. Sometimes the light is further reflected before emerging and this gives rise to a secondary rainbow seen above the first one.
Q.8: How does the emission of light occur by an atom?
Ans:
Besides sunlight, we obtain light from many sources, for example from solids and gases. Since light consists of electromagnetic waves and all substances are made up of atoms:
In atoms, electrons are moving in certain orbits. The energy of the electrons in each orbit is well defined. The orbit closer to the nucleus has less energy compared with the orbit away from it. When an electron in any orbit is excited, it jumps from the orbit at lower energy to an orbit at higher energy. However, it can’t remain in the excited state for long, so it jumps back to its parent orbit and radiates energy in the form of photons equal to the difference of those two energy levels. These photons cause light to produce and thus light is emitted by an atom.
Q.9: What is a spectrum?
Ans:
A band of colors formed by a prism or by a rainbow is called a spectrum. It has seven colors as red, orange, yellow, green, blue, indigo, and violet (VIBGYOR).
Q.10: Define electromagnetic spectrum?
Ans:
Light is electromagnetic in nature, so the spectrum formed by electromagnetic radiations is known as the electromagnetic spectrum. It consists of radio waves, microwaves, etc.
Q.11: Which waves are there in the spectrum of electromagnetic radiation? And what are their frequencies?
Ans: Waves of Electromagnetic Spectrum:
Radio Waves:
Radio waves are electromagnetic waves with a large range of wavelength from a few millimeters to several meters.Microwaves:
Microwaves are radio waves with shorter wavelengths between 1 mm and 300 mm. Microwaves are used in radars and microwave ovens.Infrared Waves:
Infrared waves are also called heat waves. These waves are radiated by hot bodies at different temperatures. The earth’s atmosphere, at a mean temperature of 250K, radiates infrared waves with a wavelength having a mean value of 10 μm.Visible Waves:
Visible waves have a wavelength range between 400 nm and 700 nm. The peak of solar radiation is at a wavelength of about 550 nm. The human eye is most sensitive to this wavelength.Ultraviolet Waves:
The wavelength of ultraviolet waves ranges from 380 nm down to 60 nm. These are emitted by hotter stars having a mean temperature greater than 25000°C.Gamma Rays:
The wavelength of gamma rays is less than m. They are emitted by the nuclei of certain radioactive substances.
Q.12: What do you understand by greenhouse?
Ans: Greenhouse:
A greenhouse is made up of glass relatively opaque to infrared radiation. Heat is trapped in the greenhouse. The term greenhouse is used for the earth's atmosphere along with the ozone layer which behaves like a greenhouse.
Q.13: What is the greenhouse effect?
Ans:
Heating of the atmosphere due to the presence of water vapors and carbon dioxide is called the greenhouse effect.
Q.14: Write down hazards of greenhouse effect?
Ans:
Heavy industries and automobiles are polluting the atmosphere, damaging the ozone layer and producing greenhouse effect.
Due to this, temperature of the earth may rise to such an extent that it will melt the polar ice caps, raising the level of the ocean, drowning all the coasts.
Q.15: What is photon?
Ans:
A photon is a packet of energy and it behaves as a particle but actually it is not a particle.
