Reflection of Light:
Light is an electromagnetic wave. Every wave has a wavelength, which means light also has a wavelength.
While traversing from the first medium into the second medium, if a part of light returns to the first medium, the phenomenon is called reflection.
Some parts of it may enter the second medium which is called refraction and some of it may be absorbed which is called absorption.
Laws of Reflection:
1st Law:
The reflected ray will be in the plane which we imagined through the incident ray and the normal.
2nd Law:
The reflected angle is equal to the incident angle.
When a light ray from the first medium to the second medium is incident on a point, first we have to imagine a perpendicular at that point which is called a normal.
Then imagine a plane that contains this incident ray and the normal.
The ray, which is incident at a point, from the first to the second medium is called the incident ray (XO). The ray which is reflected (OX') is called the reflected ray.
The angle between the incident ray and the normal is called the incident angle and the angle between the reflected ray and the normal is the reflected angle.
Q.1. If a plane mirror is considered a spherical convex mirror than what is its focal length?
Ans: Infinite.
Q.2. Where are convex mirrors used?
Ans: cars
Q.3. How measures of sensitivity of eyes can be determined?
Ans: When light rays from various objects fall on our eyes, we see those objects. The retina of eye sends the signal to the brain and the brain creates the sensation of vision.
Light is an electromagnetic wave. So, it has a definite wavelength. The wavelength can vary from a few kilometers to even nanometers. We can see a very small part of the spectrum of light. If the wavelength of the electromagnetic wave lies between 400 nm to 700 nm we can see it and we call it light. Various colour of light has various wavelength. Violet has the shortest wavelength and red has the longest wavelength among the visible light. Everything we see lies in between these two extreme colours.
Q.4. Why are the danger signals shown using red colours through human eyes can see yellowish-green the most?
Ans: Use of red light is very important in order to indicate danger. As the wavelength of red colour is high, its intensity is high. So, it can be identified from a far distance compared to other colours.
That's why danger signals are shown using red colours through human eyes can see yellowish-green the most.
Q.5. In mirrors, right and left interchanges, why top and bottom do not interchange?
Ans: Always virtual and erect image is formed in a mirror. But right and left side gets interchanged. That is, the image remains erect although the sides get interchanged.
That's why in a mirror the top and bottom do not interchange although right and left interchange.
Q.6. Why colors cannot be seen under moonlight?
Ans: When the sunlight falls on the surface of moon, all coloured rays get absorbed except for few definite rays. There fore when moonlight falls on an object having different colours, it gets absorbed. Hence, colours cannot be seen in moonlight.
Q.7. Why concave mirrors are used in the telescope used by astronomers?
Ans: The major use of cave mirrors is in telescopes. Concave mirrors are used in the largest and the smallest telescopes of the world. To create a large image of the small objects is not the only function of a good telescope. The function of a good telescope is also to create a distinct image in very dim light. That is why the bigger the concave mirror is, the more light it can converge to create a clear image.
Therefore, concave mirrors are used in the telescopes used by astronomers.
Q.8. What is mirror?
Ans: A mirror is a smooth surface on which regular reflection takes place. It is made by applying a thin layer of silver or mercury on a smooth surface. The surface opposite to the coated surface works as the reflecting surface. A clear image of an object placed in front of a mirror is formed due to reflection of light.
Q.9. What is called an image?
Ans: The reflection of an object seen at the new position due to the presence of mirror is called the image of that object.
Q.10. An object is kept at 30cm away from a concave mirror with a focal length of 20cm.
a) What is optical center?
Ans: Optical center is a point on the main axis of a lens that is so located that any ray of light passing through it suffers no deviation.
b) Find the magnification of the object in the stem.
Ans: Focal length is positive in concave mirror
f = 20cm Given that,
1/f = 1/u + 1/v Focal length, f = 20cm
1/20 = 1/30 + 1/v Distance of object, u = 30cm
v = 60cm
Magnification , m = - v/u = - 60/30 = -2 Ans.
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