Light: Reflection & Refraction - 1 - Grade 9 MCQ

If the lower half (or a part) of a mirror is covered with a black paper then a complete image of the object will be formed as before. However, the image will be less bright now.

A full length image of a distant tall building can definitely be seen by using a convex mirror because a convex mirror always forms a virtual, erect and highly diminished image.

The angle of incidence of any light ray passing through the centre of curvature of a spherical mirror is 0°, because a line joining centre of curvature to any point on the mirror is a normal drawn at that point of the mirror.

• The magnification (m) produced by the mirror is given by the ratio of the height of the image to the height of the object: m = height of image / height of object = 5 mm / 10 mm = 0.5
• For a real, inverted image formed by a concave mirror, both the image distance (v) and object distance (u) are negative (according to the sign convention).
• The image is formed at 30 cm in front of the mirror, so v = -30 cm.
• The magnification formula for mirrors is:
  m = -v/u
  So, 0.5 = -(-30)/u
  0.5 = 30/u
  u = 60
• According to the sign convention, object distance u = -60 cm.
  Now, using the mirror formula:
  ​​​​​​​
• So, f = -20 cm 
• Therefore, the focal length of the mirror is -20 cm.

In optics, a real and inverted image formed by a concave mirror has a negative magnification. This is because:

• Real Images are formed on the same side as the object.
• These images are inverted, indicating a negative value for magnification.

As light rays from sun (u = ∞) converge at a point 15 cm in front of a concave mirror, hence focal length of concave mirror f = - 15 cm.
To form an image of exactly same size as that of an object, the object should be placed at the centre of curvature (u = R = 2f) of mirror. Hence, the object should be placed at 30 cm in front of mirror.

The correct answer is:

d) Between F and 2F

Explanation:

A convex lens forms a magnified real image when the object is placed between the focal point (F) and twice the focal length (2F).

Key Points:

• Real image: Formed when light rays actually converge (can be projected on a screen).

• Magnified: The image is larger than the object.

• Positioning:

  • If the object is at F, no image forms (rays become parallel).

  • If the object is at infinity, the image forms at F (diminished and real).

  • If the object is at 2F, the image is same size and real (not magnified).

  • Between F and 2F: Image is real, inverted, and magnified.

Final Answer:

d) Between F and 2F

A concave mirror forms a virtual, erect and magnified image when an object is placed between pole and focus point of the mirror. As focal length of given concave mirror, hence object must be placed at a distance less than 12 cm i.e., u < 12 cm. Thus, the object may be placed at a distance of 10 cm from the mirror.

• (a) Always virtual, erect, and diminished: Incorrect (Only true when the object is very close to the mirror).

• (b) Always virtual, inverted, and diminished: Incorrect (Concave mirrors never form virtual AND inverted images).

• (c) Sometimes real, inverted, and diminished: Correct (This happens when the object is placed beyond centre).

• (d) Always real, inverted, and magnified: Incorrect (Only true when the object is between Centre and Focus).

Since the image can be real, inverted, and diminished in some cases (object beyond Centre), option (c) is the most accurate.

Answer: (c) Sometimes real, inverted and diminished