iGCSE Physics (0625) 3.2.1 Reflection of light-Exam Style Questions- New Syllabus
▶️ Answer/Explanation
Detailed solution:
According to the law of reflection for a plane mirror, the angle of incidence i is always equal to the angle of reflection r.
This relationship can be expressed by the linear equation r=i, which follows the form y=mx+c where the gradient m=1 and the intercept c=0.
Since r is directly proportional to i, the resulting graph must be a straight line passing through the origin (0,0).
As the angle of incidence increases, the angle of reflection increases at the exact same rate, maintaining a constant ratio.
Graph C correctly depicts this proportional relationship as a straight line with a constant positive gradient.
Options A, B, and D are incorrect because they represent non-linear or inverse relationships that violate the law of reflection.
▶️ Answer/Explanation
Detailed solution:
All angles in optics are measured between the light ray and the normal, which is the dashed line perpendicular to the surface.
The angle of reflection is the angle between the reflected ray and the normal in the first medium; in the diagram, this is identified as angle $1$.
The angle of refraction is the angle between the refracted ray and the normal in the second medium; in the diagram, this is identified as angle $4$.
Angles $2$ and $3$ are incorrect because they are measured from the ray to the glass surface (the interface) rather than the normal.
According to the law of reflection, the angle of incidence equals the angle of reflection ($i = r$).
Therefore, the correct row identifying the required angles is B.
▶️ Answer/Explanation
Detailed solution:
In a plane mirror, the image distance $d_i$ is always equal to the object distance $d_o$ from the mirror.
Initially, the person is $1.0\text{ m}$ from the mirror, so the image is also $1.0\text{ m}$ behind it, making the total distance $2.0\text{ m}$.
After $1\text{ s}$, the mirror moves $1.0\text{ m}$ away, so the person is now $2.0\text{ m}$ from the mirror.
The new image forms $2.0\text{ m}$ behind the mirror, resulting in a total distance of $4.0\text{ m}$ from the person.
The image distance from the person increased from $2.0\text{ m}$ to $4.0\text{ m}$ in $1\text{ s}$.
Therefore, the image moves away from the person at a speed of $2.0\text{ m/s}$.