Olympiad Test: Light, Shadows & Reflections - Class 6 MCQ

The image described in the question represents a periscope, which uses plane mirrors for multiple reflections. This device is commonly found in submarines.

Key points about the image formation in a plane mirror:

• The image is always virtual, meaning it cannot be projected onto a screen.
• The image appears upright and retains the same shape and size as the object.
• The distance of the image from the mirror is equal to the distance of the object from the mirror.

In this scenario:

• The image of object O is first seen at position b after the first reflection.
• After the second reflection, the person perceives the image at position d.

A translucent object allows some light to pass through it, but not enough to see the full details or clear image of the source. When Abhishek looked at the lighted torch through the object, he could see a faint glow but not the torch itself. This suggests that the object is translucent, as it diffuses the light and prevents a clear image from being seen.

• A transparent object would allow the full image of the torch to be seen clearly.
• An opaque object would block all light, preventing any glow or image from being seen through

All of the provided statements explain why air is not visible:

• A: It is nearly a perfect transparent substance.
  Air is transparent, allowing light to pass through without significant absorption or scattering, making it invisible to the naked eye.
• C: It transmits whole of light.
  Air effectively transmits light, enabling it to pass through without distorting or scattering enough to make the air visible.
• D: It neither absorbs nor reflects light.
  Air does not absorb or reflect light in a way that would make it visible; instead, it allows light to pass through.

Since all the statements are correct, B: All of these is the best answer.

When a person moves closer to a plane mirror, the size of their image in the mirror does not change.

This phenomenon occurs because:

• The size of the image in a plane mirror is always the same as the size of the object.
• This remains true regardless of the distance between the object and the mirror.

Therefore, the correct conclusion is that the size of the image will remain the same.

Book is an opaque object and hence does not allow the light of torch to pass through it. As there is no light on the ring, hence no image will be formed and the screen will remain dark due to absence of any light ray.

The glass does not create a dark shadow when light shines on it because it is a transparent object. This means that:

• Light can easily pass through glass.
• Shadows are formed when an opaque object blocks light.
• Since glass allows light to pass without obstruction, it prevents the formation of a shadow.

In summary, the transparency of glass is the reason it does not cast a shadow.

• A: The image formed by a pinhole camera is inverted because light travels in a straight line.
  This is true. In a pinhole camera, light travels in straight lines through the small hole (pinhole) and projects an inverted image on the opposite side of the box.

• B: Light does not change its direction on reflection.
  This is false. Light does change direction when it reflects off a surface, following the law of reflection (the angle of incidence equals the angle of reflection).

• C: As we move the object away from the light source, the shadow of the object becomes smaller.
  This is false. The shadow becomes larger as the object moves farther away from the light source, because the light rays diverge more.

• D: Reflection is possible only from unpolished surface.
  This is false. Reflection can occur on both polished and unpolished surfaces, but the type of reflection (specular or diffuse) depends on the smoothness of the surface. Polished surfaces cause specular reflection, which creates clear, sharp reflections.

Thus, the correct answer is A.

The umbra, the region of complete darkness, does not reach the earth. This is because:

• The umbra creates a shadow that is too small to be seen on the ground.
• The penumbra, which is the area of partial brightness, is much larger.
• This partial light is too faint to be visible to us.

Thus, while objects like kites and aeroplanes move in space, their shadows do not appear on the earth.

Figure (iii) is a periscope, which allows you to see around corners. It operates on the principle of rectilinear propagation and reflection of light.

A periscope is an optical instrument designed to view objects that are otherwise obstructed or above the direct line of sight.

• It consists of two mirrors placed at an angle.
• The mirrors reflect light from the object to your eyes.
• This enables you to see around obstacles without moving.

- Mirror Size and Position: To see your full image, the mirror must be at least half your height.
- Action I (Move Away): Moving away increases the field of view, allowing you to see more of yourself. Thus, moving away can help you see your full image if the mirror is tall enough.
- Action III (Use Taller Mirror): A taller mirror ensures you see your entire reflection regardless of distance.
- Conclusion: Either moving away (I) or using a taller mirror (III) will allow you to see your full image. Thus, the correct answer is C.

The ability to see the level of water in a cup depends on the transparency of the material.

• Glass cup: Glass is transparent, which means Vijay will be able to clearly see the level of water inside the glass cup.
• Steel cup: Steel is opaque, so Vijay will not be able to see through it.
• Porcelain cup: Porcelain is usually opaque or semi-translucent, so it will not allow clear visibility of the water level.
• Frosted glass cup: Frosted glass is translucent, meaning it diffuses light and partially blocks visibility, so Vijay will not see the water level as clearly as in a regular glass cup.

Thus, the correct answer is A: Glass cup.

• A metal plate: Metal is opaque, so it does not allow light to pass through.
• A glass tumbler: Glass is transparent, meaning it allows light to pass through easily.
• A book: A book is opaque, so it does not allow light to pass through.
• A wooden block: Wood is also opaque, preventing light from passing through.

Thus, the correct answer is B: A glass tumbler.

The length of the shadow of a stick depends on the position of the sun in the sky. At different times of the day, the angle of the sun changes, which in turn affects the length of the shadow.

• At 8:15 am, the sun is relatively low in the sky, creating a longer shadow.
• At 12:00 noon, the sun is at its highest point in the sky (in the case of most locations, it is directly overhead or near overhead). At this time, the shadow of the stick will be much shorter because the angle between the stick and the sun is very small.

So, the shadow at 12:00 noon will be shorter than the shadow at 8:15 am.

The correct answer is: (a) It will be shorter than one at 8:15 am.

All the rays that emerge from an object placed at the centre of a transparent sphere will be normal to the surface of the sphere.

This means:

• The apparent position of the dot will appear the same as its actual position.
• Light travels in straight lines, so the dot is not distorted by the sphere.
• As a result, the observer sees the dot directly in line with its actual location.

Transparent material allows light to pass through, while opaque objects block it. Translucent materials permit some light to pass through. In this scenario:

• The bright spot is visible because the object is opaque.
• Shadows are created when an opaque object is present.

Specular reflection occurs when parallel rays of light hit a smooth surface, and after reflection, the rays remain parallel to each other. This type of reflection results in clear, well-defined images (like in a mirror), because the reflective angles are all the same for each incident ray.

• Diffused reflection: This happens when light strikes a rough surface, scattering the rays in many directions, which results in a blurry or unclear image.
• Mirror reflection: This is essentially a form of specular reflection but specifically refers to the reflection from a mirror's smooth surface.
• Lake reflection: This is an example of specular reflection that occurs on the surface of water, but it is not a distinct type of reflection.

Thus, the correct answer is A: Specular reflection.

When light reflects off an object, it creates images that we can see. The clarity of these images depends on the surface of the object.

Here are some key points about how images are formed on surfaces:

• Images are clearer on smoother and brighter surfaces.
• When light hits a smooth surface, it reflects back, allowing us to see the object.
• Highly polished surfaces, like glass, reflect most of the light that strikes them.
• A polished glass surface, especially in a colour like red, can reflect light effectively.

In the case of Riya and Priya:

• They were seated around a round table.
• The table's polished surface reflected the light from them.
• This reflection allowed them to see their own images and each other's images on the table.

Thus, the correct answer is that the table top is made of polished glass.

A pinhole camera works by allowing light to pass through a tiny hole (the aperture) and project an image on the opposite side of the camera. It does not use a lens, but the small hole (aperture) serves to focus the image.

• B: Has a lens, but no aperture: This is incorrect, as a pinhole camera does not have a lens.
• C: Has no lens and no aperture: This is incorrect, as the pinhole serves as the aperture.
• D: Has a self-timer: This is incorrect, as a pinhole camera typically does not include such features.

Thus, the correct answer is A: Has an aperture, but no lens.

A coloured plastic bottle is an opaque material. When light hits it, a shadow will form on the screen. In contrast, light can pass through the transparent sheet.

The materials that allow only some light to pass through are known as translucent materials. These materials let light through partially, which means they do not create a clear shadow.

• A: A cylindrical object can cast a rectangular as well as a circular shadow.
  This is true. Depending on the angle of the light source and the orientation of the cylindrical object, it can cast both rectangular (if the object is viewed from the side) and circular (if viewed from the top) shadows.

• B: Polished surface produces a clear image.
  This is true. A polished surface causes specular reflection, which creates clear and sharp images, as it reflects light in parallel directions.

• C: The umbra is the region of a shadow which is grey in colour.
  This is false. The umbra is the darkest part of a shadow, where the light source is completely blocked. It is typically black or very dark, not grey. The penumbra is the lighter, greyish region of the shadow, where some, but not all, of the light is blocked.

• D: Light travels in a straight line.
  This is true. In the absence of obstacles or medium changes, light travels in a straight line.

Thus, the statement that is not true is C: The umbra is the region of a shadow which is grey in colour.