4.
When viewed in sunlight, a flower has red petals and a green stalk. You pluck the flower
and take it to a room illuminated with a red lamp. What colours does the flower now
appear to be?
a. Black petals and a green stalk
b. Black petals and a black stalk
c. Red petals and a black stalk
d. Red petals and a green stalk
The correct answer is c. Red petals and a black stalk. In sunlight (white light), the red petals reflect red wavelengths while absorbing others, and the green stalk reflects green wavelengths while absorbing others. Under a red lamp, which emits only red light, the red petals reflect that light and remain visible as red, but the green stalk absorbs red light (since it doesn’t reflect it) and appears black.
Option Analysis
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a. Black petals and a green stalk: Incorrect, as red petals reflect red light, so they won’t appear black; the stalk can’t reflect green since no green light is present.
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b. Black petals and a black stalk: Incorrect, because red petals specifically reflect red light from the lamp.
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c. Red petals and a black stalk: Correct, matching the selective reflection principle under monochromatic red light.
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d. Red petals and a green stalk: Incorrect, as the stalk requires green light to appear green, which the red lamp lacks.
In sunlight, a flower with red petals and green stalk dazzles with vibrant colors, but what happens under a red lamp? This flower color change red lamp puzzle tests light reflection principles key for biology and physics exams. Understanding selective absorption reveals the science behind everyday observations.
Light Reflection Basics
Objects appear colored due to reflected wavelengths. Red petals reflect ~650 nm red light, absorbing blue/green; green stalks reflect ~550 nm green, absorbing red/blue. White sunlight contains all visible wavelengths (400-700 nm), enabling true colors.
Under a red lamp (monochromatic ~650 nm), only red light illuminates:
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Red petals reflect it, staying red.
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Green stalk absorbs it fully, appearing black (no reflection to eyes).
Exam MCQ Breakdown
This question mimics CSIR NET-style optics-biology crossovers:
| Option | Petals | Stalk | Why Wrong/Correct |
|---|---|---|---|
| a | Black | Green | Petals reflect red; no green light for stalk |
| b | Black | Black | Petals must reflect red light |
| c | Red | Black | Matches physics: reflection/absorption |
| d | Red | Green | No green light available |
Real-World Applications
Flower color change red lamp demos aid teaching pigmentation (chlorophyll absorbs red/blue for photosynthesis). Similar in stage lighting or plant growth labs—red light boosts flowering via phytochromes but alters visuals. Experiment: Test with filters for hands-on learning.
