Lecture- 6 : CLASS-X: SCIENCE : Chapter: REFLECTION OF LIGHT & SIGN CONVENTIONS

CLASS X   |    SCIENCE    |    LIGHT

      Notes prepared by Subhankar Karmakar

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click to access other class notes

SCIENCE: CLASS X: CBSE (CLASS NOTES)

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SIGN CONVENTION FOR SPHERICAL MIRRORS:

According to the new cartesian sign convention:

1. All the distances are measured from pole of the mirror as origin. 

2. Distances measured in the same direction as that of incident light are taken as positive.

3. Distances measured against the direction of incident light are taken as negative.

4. Distances measured upward and perpendicular to the principal axis are taken as positive.

5. Distances measured downward and perpendicular to the principal axis are taken as negative. 

 

KEY POINTS TO REMEMBER

• The object is always placed on the left side of the mirror. 

• All the distances measured from the pole (P) of mirror to the right side will be considered positive and to the left side will be negative. 

• The object distance (u) is always negative.

• If an image is formed behind a concave mirror or to the right side of the mirror, the image distance (v) is positive, if the images formed in front of the mirror or to the left side of the mirror, then the image distance will be negative. 

• The image distance (v) for a convex mirror will be always positive.

• The focal length of a concave mirror is always negative

• The focal length of a convex mirror is always positive

• The height of an object is always positive

• If an image is formed above the principal axis its height is positive

• If an image is formed below the principal axis its height is negative

• The height of all the virtual and erect images is positive

• The height of all the real and inverted images is negative.

MIRROR FORMULA:

A formula which gives the relationship between image distance (v), object distance (u) and focal length (f) of a spherical mirror is known as the mirror formula. It is given as

1/v + 1/u = 1/f

Linear magnification produced by mirrors:

The ratio of the height of image to the height of object is known as linear magnification. It is also equal to the ratio of the image distance to the object distance, with a minus sign. 

∴  magnification = height of image/height of object

⟹ m = h₂ / h₁ = - v/u

h₁ = height of object

h₂ = height of image

• if the magnification has a plus sign, then the image is virtual and erect. 

• if the magnification has a negative sign, then the image is real and inverted. 

Position of the image means image distance.

Nature of image means whether  the image is "real and inverted" or "virtual and erect".

Size of image means value of magnification.

Lecture- 5 : CLASS-X: SCIENCE : Chapter: Reflection of light & concave mirror

CLASS X   |    SCIENCE    |    LIGHT

      Notes prepared by Subhankar Karmakar

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click to access other class notes

SCIENCE: CLASS X: CBSE (CLASS NOTES)

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 • Rules for obtaining images formed by Concave Mirror:

The image will be formed at the point where atleast two reflected rays intersect or appear to intersect. 

1. A ray of light which is is parallel to the principal axis of a concave mirror, passes through its focus after reflection from the mirror. 

2. A ray of light passing through the centre of curvature of a concave mirror is reflected back along the same path. Arrow pointing from left to right indicates the direction of incident ray and the arrow pointing from right to left indicates the direction of reflected ray.

3. A ray of light passing through the focus of a concave mirror becomes parallel to the principal axis after reflection.

4. A ray of light which is incident at the pole of a concave mirror is reflected back making the same angle with the principal axis. 

• If a ray of light is incident on a concave mirror along its principal axis, then it is reflected back along the same path. 

• FORMATION OF DIFFERENT TYPES OF IMAGES BY A CONCAVE MIRROR

The type of image formed by a concave mirror depends on the position of object in front of the mirror. At different places, an object produces different types of images. 

a. When the object is in between Pole (P) and focus (F):

When an object is placed between the pole (P) and focus (F) of a concave mirror, the image formed is:

i. Behind the mirror

ii. Virtual and erect, and

iii. Larger than the object or magnified.

Uses of concave mirror using this type of images:

1. A concave mirror can be used to magnify objects. Therefore, it will be used as a magnifying glass.

2. A concave mirror can be used as a makeup mirror. It magnifies a part of the face.

3. Dentist's mirror is a small concave mirror fitted in a frame with a long handle. It gives magnified image of tooth.

b. When the object is placed at the focus (F) of a concave mirror:

When an object is placed at the focus of a concave mirror, the image formed is:

i. At infinity, 

ii. Real and inverted, and

iii. Highly magnified.

Uses of concave mirror using this type of images:

1. When a light bulb is placed at the focus of a concave mirror reflector, the diverging light rays off the bulb are collected by the concave reflector and then reflected to produce a strong, parallel beam of light. 

c. When the object is placed between focus (F) and centre of curvature (C):

When an object is placed between the focus (F) and the centre of curvature (C) of a concave mirror, the image formed is:

i. Beyond the centre of curvature

ii. Real and inverted, and

iii. Larger than object or magnified.

d. When the object is placed at the centre of curvature (C) of a concave mirror:

When an object is placed at the centre of curvature (C) of a concave mirror, the image formed is:

i. At the centre of curvature (C),

ii. Real and inverted, and

iii. Same size as the object.

e. When the object is is beyond the centre of curvature (C) of the concave mirror:

When an object is placed beyond the centre of curvature (C) a concave mirror, the image formed is:

i. Between the focus (F) and the centre of curvature (C),

ii. Real and inverted, and

iii. Smaller than the object or diminished.

f. When the object is at infinity:

When an object is at infinity from a concave mirror, the image found is:

i. At the focus (F), 

ii. Real and inverted, and

iii. Much smaller than the object or highly diminished. 

"This means that a concave mirror can concentrate all The parallel rays of light to its focus."

Uses of concave mirror using this type of images:

1. A concave mirror is used as a "head mirror" by the doctors to concentrate light coming from a lamp onto the body part of a patient like ear, nose, throat etc. to be examined. 

2. The concave "metal dishes" are used in dish antenna of televisions to receive TV signals from the very distant communication satellite which are high up in the sky. 

USES OF CONCAVE MIRRORS:

1. A concave mirror can be used to magnify objects. Therefore, it will be used as a magnifying glass.

2. A concave mirror can be used as a makeup mirror. It magnifies a part of the face.

3. Dentist's mirror is a small concave mirror fitted in a frame with a long handle. It gives magnified image of tooth.

4. When a light bulb is placed at the focus of a concave mirror reflector, the diverging light rays off the bulb are collected by the concave reflector and then reflected to produce a strong, parallel beam of light. 

5. A concave mirror is used as a "head mirror" by the doctors to concentrate light coming from a lamp onto the body part of a patient like ear, nose, throat etc. to be examined. 

6. The concave "metal dishes" are used in dish antenna of televisions to receive TV signals from the very distant communication satellite which are high up in the sky.