Reflection
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AP Physics 2 › Reflection
A pulse on a taut string reaches a rigid wall. The incident pulse travels to the right toward the wall, and the reflected pulse travels to the left away from the wall. Which statement correctly describes the reflection at the rigid boundary?
The reflected pulse amplitude must increase because the wall is rigid.
The reflected pulse is not inverted because the string’s speed is unchanged.
The reflected pulse travels along the wall instead of back on the string.
The reflected pulse is inverted relative to the incident pulse.
Explanation
This question tests understanding of reflection of mechanical waves at boundaries. When a wave pulse on a string encounters a rigid boundary (fixed end), the reflected pulse is inverted relative to the incident pulse because the boundary cannot move. The string must have zero displacement at the wall, so when the incident pulse tries to lift the string upward at the boundary, the reflected pulse must pull it downward to maintain this condition. Choice B incorrectly claims no inversion occurs, confusing wave speed (which doesn't change) with the boundary condition that determines pulse orientation. For mechanical wave reflections, remember that rigid boundaries invert pulses while free boundaries do not.
A light ray reflects from a flat mirror. The incident ray is directed downward and to the right toward the mirror, and the reflected ray leaves upward and to the right. Which statement correctly describes the reflection?
The reflected ray must be parallel to the surface for any smooth mirror.
The normal bisects the angle between the incident and reflected rays.
The mirror surface bisects the angle between the incident and reflected rays.
The reflected angle increases when light slows down near the mirror.
Explanation
This question tests understanding of reflection. The law of reflection requires that the angle of incidence equals the angle of reflection, both measured from the normal. This means the normal line bisects the angle between the incident and reflected rays—it acts as the axis of symmetry for the reflection. The incident ray, normal, and reflected ray all lie in the same plane. Choice B incorrectly suggests the mirror surface bisects this angle, which would violate the law of reflection except when rays are perpendicular to the surface. Visualize reflection as a symmetric process about the normal, not about the surface.
A light ray reflects from a smooth, horizontal mirror. The incident ray travels down and to the right, and the reflected ray travels up and to the right. Which statement correctly describes the relationship between the incident and reflected angles?
They are equal when measured from the surface itself.
They are equal when measured from the normal to the surface.
They differ because the mirror changes the light’s speed.
They differ because reflection depends on the light’s frequency.
Explanation
This question tests understanding of reflection angle measurement. The law of reflection states that the incident angle equals the reflected angle when both are measured from the normal (the line perpendicular to the surface at the point of incidence). The incident ray traveling down and to the right and the reflected ray traveling up and to the right make equal angles with the normal, which points vertically from the horizontal mirror. Choice B incorrectly suggests measuring angles from the surface itself, a common error that violates the law of reflection. Always identify the normal first—it's perpendicular to the surface—then measure both angles from this reference line.
A light ray in air strikes a flat mirror. The incident ray travels up and to the left and makes an angle $\theta_i$ with the normal. The reflected ray leaves traveling up and to the right. Which statement correctly describes the reflection?
The reflected angle equals $90^\circ-\theta_i$ measured from the normal.
The reflected angle depends on the mirror’s material and thickness.
The reflected ray must travel along the mirror when $\theta_i\neq 0$.
The reflected angle equals $\theta_i$ measured from the normal.
Explanation
This question tests understanding of reflection angles. The law of reflection states that the angle of incidence equals the angle of reflection, with both angles measured from the normal to the reflecting surface. Since the incident ray makes angle θᵢ with the normal, the reflected ray must also make angle θᵢ with the normal, traveling on the opposite side. Choice B incorrectly suggests the reflected angle is 90°-θᵢ, which would be the complementary angle—this error often arises from measuring angles relative to the surface instead of the normal. Always establish the normal at the point of incidence and measure both incident and reflected angles from this reference line.
A water wavefront strikes a straight, rigid barrier and reflects. The incident wave travels toward the barrier at an angle to the normal, and the reflected wave travels away on the other side of the normal. Which statement correctly describes the reflection?
The reflected wave travels along the barrier for any nonzero incident angle.
The reflected angle increases because the barrier is rigid.
The reflected wavefront makes the same angle to the barrier as the incident wavefront.
The reflected wavefront makes the same angle to the normal as the incident wavefront.
Explanation
This question tests understanding of reflection of water waves. The law of reflection applies to all wave types: the angle of incidence equals the angle of reflection, with both angles measured from the normal to the reflecting surface. When a water wavefront strikes a barrier at an angle, the reflected wavefront makes the same angle with the normal as the incident wavefront, traveling on the opposite side of the normal. Choice A incorrectly refers to angles measured from the barrier rather than the normal, a common misconception that leads to incorrect angle relationships. For any wave reflection, establish the normal (perpendicular to the barrier) and measure all angles from it.
A sound pulse in air reflects from a large, flat concrete wall. The incident pulse travels toward the wall, and the reflected pulse travels back into the room. Which statement correctly describes the reflection?
The reflected pulse returns at a larger angle because concrete is denser than air.
The reflected direction depends on the speed of sound at the wall surface.
The reflected pulse returns with the same angle to the normal as the incident pulse.
The reflected pulse must travel along the wall due to friction with the surface.
Explanation
This question tests understanding of reflection of sound waves. The law of reflection applies to all types of waves, including sound waves: the angle of incidence equals the angle of reflection, both measured from the normal to the reflecting surface. When a sound pulse reflects from a flat wall, it returns at the same angle to the normal as it arrived, regardless of the wall material's density. Choice B incorrectly claims the angle increases due to the concrete's density, confusing reflection (which follows a simple geometric law) with refraction (where material properties matter). For any wave reflection from a smooth surface, the incident and reflected angles are always equal when measured from the normal.
A light ray reflects from a smooth surface. The incident ray approaches making $15^\circ$ with the normal; the reflected ray leaves on the opposite side of the normal. Which statement correctly describes the reflection?
The reflected ray travels along the surface since the incident angle is small.
The reflected angle is $75^\circ$ because angles are measured from the surface.
The reflected angle is $15^\circ$ when measured from the normal.
The reflected angle depends on the surface material’s color.
Explanation
This question tests understanding of reflection. According to the law of reflection, when the incident ray approaches at 15° from the normal, the reflected ray must leave at 15° from the normal on the opposite side. This angle relationship is independent of surface color, material properties, or the size of the angle. Choice A incorrectly measures from the surface instead of the normal - if the ray makes 15° with the normal, it makes 75° with the surface, but we always use normal angles for the reflection law. The fundamental principle to remember is that reflection angles are always measured from the normal, and the angle in equals the angle out.
A plane wave on a string reaches a rigid boundary and reflects. The incident pulse travels right; the reflected pulse travels left. Which statement correctly describes the reflection behavior?
The reflected pulse returns along the same line, reversing direction.
The reflected pulse must travel along the boundary after reflection.
The reflected pulse direction changes only if the string tension changes.
The reflected pulse angle depends on wave speed in the wall material.
Explanation
This question tests understanding of reflection. When a wave pulse on a string reaches a rigid boundary, it reflects back along the same line but in the opposite direction - this is a special case of the reflection law where the angle of incidence is 0° (normal incidence). The pulse travels right, hits the boundary perpendicularly, and returns left along the same path. The wave speed in the wall material is irrelevant because the wave doesn't enter the wall; it reflects at the boundary. Choice D incorrectly suggests that string tension affects the reflection direction, but tension only affects wave speed, not reflection angles. For normal incidence (perpendicular approach), remember that the reflected path retraces the incident path in reverse.
A sound wave reflects from a large, flat concrete wall. The incident direction and reflected direction make equal angles with the normal to the wall. Which statement correctly describes the reflection?
The reflected angle equals the incident angle, both measured from the normal
The reflected wave travels parallel to the wall regardless of incidence angle
The reflected angle is larger because sound travels slower near the wall
The reflected angle depends on the wall material and is not predictable
Explanation
This question tests understanding of reflection. The law of reflection applies to all types of waves, including sound waves, stating that the angle of incidence equals the angle of reflection when measured from the normal to the reflecting surface. This law holds regardless of the wave type or the material of the reflecting surface, as long as the surface is smooth compared to the wavelength. Choice B incorrectly suggests the angle depends on wave speed near the wall, confusing reflection with refraction. When analyzing any wave reflection, apply the same geometric principle: incident angle equals reflected angle, both measured from the normal.
A water wavefront approaches a straight seawall at an angle. The incident direction is perpendicular to the incoming wavefronts; the reflected direction is perpendicular to the reflected wavefronts. Which statement correctly describes the reflection?
The reflected direction makes a larger angle because water waves slow near the wall
The reflected direction is always along the wall regardless of incidence
The angle between the incident direction and the normal equals the angle between the reflected direction and the normal
The reflected direction depends on wave amplitude, not on the incident direction
Explanation
This question tests understanding of reflection. When water waves reflect from a straight seawall, the law of reflection applies: the angle between the incident direction and the normal equals the angle between the reflected direction and the normal. The incident and reflected directions are perpendicular to their respective wavefronts, maintaining the same angular relationship with the normal. Choice B incorrectly suggests that wave speed affects the reflection angle, confusing reflection with refraction phenomena. For any wave reflection from a straight barrier, visualize the wavefronts as parallel lines and remember that the perpendicular directions follow the equal-angle rule.