Concept Cracker
Define principal axis, focus, and pole for curved mirrors.
Principal Axis: The principal axis is an imaginary straight line that passes through the pole of the mirror and its center of curvature. It acts as the line of symmetry for the spherical mirror and is perpendicular to the mirror's surface at the pole.
Focus (or Principal Focus):
Explain why light reflection is regular on a mirror surface but diffused on paper.
Light reflection is regular (or specular) on a mirror surface because the surface is very smooth and polished at a microscopic level. This smoothness causes the light rays to reflect in a single, predictable direction, producing a clear image.
In contrast, paper has a rough and uneven surface with many tiny irregularities. When light hits paper, it reflects in many different directions, causing diffused reflection. This scattered reflection prevents a clear image from forming and makes the paper appear matte.
Where is the image formed when an object is at the center of curvature (C)?
For a concave mirror, when an object is placed at the center of curvature (C), the image is formed:
Why is the sun’s heat focused using a concave mirror in solar cookers?
Concave mirrors are used in solar cookers because they converge parallel rays of sunlight (which is how sunlight effectively reaches Earth) to a single focal point. Placing food at this focus concentrates a large amount of solar energy into a small area, generating enough heat to cook efficiently.
A concave mirror forms an image at 30 cm and focal length is 15 cm. Find the object distance.
u=−30 cm.
on same side of the object, 30 cm away from the mirror.
Define the laws of refraction with a diagram.
Laws of Refraction:
First Law: The incident ray, the refracted ray, and the normal to the interface of the two transparent media at the point of incidence, all lie in the same plane.
Second Law (Snell's Law): For a given pair of media and for light of a specific color, the ratio of the sine of the angle of incidence (θ1) to the sine of the angle of refraction (θ2) is a constant. This constant is known as the refractive index of the second medium with respect to the first, denoted by n21 (or n2/n1).
Mathematically, Snell's Law is expressed as: n1sinθ1=n2sinθ2 or sinθ2/sinθ1=n1/n2=constant
Why does a coin appear raised when placed under water?
A coin appears raised in water due to refraction. Light rays from the coin bend away from the normal as they pass from denser water to rarer air. Your brain, assuming light travels straight, traces these bent rays back, making the coin appear at a shallower, "raised" position.
Describe image formed when object is placed at F in a concave mirror.
At infinity, highly enlargred.
Why do makeup mirrors produce enlarged images?
Makeup mirrors are concave mirrors. They produce enlarged images when your face is placed within their focal length, resulting in a magnified, virtual, and upright image.
A convex mirror forms a virtual image 6 cm behind mirror with focal length as 10 cm. Find object distance.
u=−15 cm.
Differentiate between real and virtual images with examples.
Real Image
Examples:
Virtual Image
Examples:
Why does light bend at the interface between glass and air?
Light bends because it changes speed as it passes from one medium (like air) to another (like glass) with a different optical density. This change in speed causes the light ray to change direction.
Image formed when object is between F and pole in concave mirror – explain.
When an object is between the focus (F) and pole (P) of a concave mirror, the image formed is virtual, upright, magnified, and located behind the mirror. This occurs because reflected light rays appear to diverge from a point behind the mirror.
Why do road safety mirrors use convex mirrors even though they give smaller images?
Road safety mirrors (like passenger-side car mirrors or security mirrors at blind spots) use convex mirrors primarily for their wider field of view, even though they make objects look smaller. This wide angle allows drivers to see a much larger area, effectively reducing blind spots and increasing overall awareness of surroundings, which is critical for preventing accidents. The trade-off of a smaller image is accepted for the benefit of seeing more.
A real image is formed 20 cm from a lens with a focal length of 10 cm. Find object distance and nature of lens.
Define “refractive index” and explain its relation with light speed.
The refractive index of a medium is a dimensionless quantity that describes how much light slows down when it passes through that medium compared to its speed in a vacuum. It is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in the medium (v).
Relation with Light Speed:
The relationship is inverse and directly proportional:
n=c/v
Why can a concave mirror form all types of images, but convex only one type?
A concave mirror is a converging mirror. Its inward curve allows it to bring light rays to a real focus, enabling it to form both real, inverted images (when the object is further away) and virtual, upright, magnified images (when the object is very close).
A convex mirror is a diverging mirror. Its outward curve always spreads light rays out. Consequently, it can only form one type of image for a real object: a virtual, upright, and diminished image, regardless of object distance.
Draw ray diagram and describe image when object is between C and F.
What is Access
Suggest a situation where a plane mirror might fail in a practical use.
A plane mirror would fail significantly in a car's rearview mirror (specifically the passenger-side one).
While a plane mirror provides a true-sized, upright image, its field of view is very limited. In a car, this would create dangerous large blind spots, preventing the driver from seeing vehicles in adjacent lanes, leading to a much higher risk of accidents when changing lanes or merging. This is precisely why convex mirrors are used on the passenger side, despite making objects appear smaller.
Using Mirror formula for u = –20 cm, f = –15 cm. Find v. Describe nature of image and which type of mirror used?
State and explain Snell’s Law. Use diagram.
For a given pair of transparent media and for light of a specific color (or frequency), the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. This constant is equal to the ratio of the refractive indices of the two media.
Mathematical Form:
n1sinθ1=n2sinθ2
Alternatively, it can be written as:
sinθ2/sinθ1=n1/n2
How does the position of an object relative to the focus (F) affect the size and nature of the image formed by a concave mirror? Explain with reasoning.
The position of the object relative to the focus (F) of a concave mirror greatly affects the size and nature of the image formed:
Object beyond C (center of curvature): The image is real, inverted, smaller than the object, and formed between F and C.
Object at C: The image is real, inverted, and same size as the object, formed at C.
Object between C and F: The image is real, inverted, and larger than the object, formed beyond C.
Object at F: No image is formed because reflected rays are parallel and never meet.
Object between F and the mirror (pole): The image is virtual, erect, and larger, formed behind the mirror.
What happens to the image as object approaches focus from beyond C in concave?
As the object moves from beyond C towards F, the real, inverted image moves away from the mirror (from between F and C, to C, and then beyond C towards infinity) and continuously increases in size, transitioning from diminished to same-sized to highly magnified.
How can we design a telescope using mirrors and lenses efficiently?
So, to make an efficient telescope, we combine them:
A lens forms image at 25 cm, object at 15 cm. Find focal length and identify lens type.
Case 1: