Spectra & Evidence for an Expanding Universe — Practice · Grade 9 Science

Chapter: Stars & the Universe

What this practice covers

By the end of Spectra & Evidence for an Expanding Universe you should be able to:

8 questions, numeric, multiple choice and written answer, each with a worked explanation.

Vocabulary used: spectrum, redshift, cosmic microwave background, light-year, megaparsec (Mpc).

Problem 1

A 400 nm rest-wavelength line is observed at 440 nm. Find z.
Answer and explanation

0.1

(440 − 400) ÷ 400 = 0.10.

Problem 2

A line at longer wavelength than its rest value is…
  1. redshifted
  2. blueshifted
  3. proof of a new element by itself
  4. unmeasurable
Answer and explanation

redshifted

Redshift refers to increased wavelength.

Problem 3

Why compare multiple spectral lines?
Answer and explanation

A pattern makes identification more reliable and helps distinguish composition from shifts or measurement errors.

Problem 4

A known line at 500 nm appears at 510 nm. Calculate z=(observed−rest)/rest. Explain the reasoning in your own words.
Answer and explanation

Redshift z=0.02; interpretation needs the full pattern and relevant model.

Wavelength difference=510−500=10 nm.

z=10/500=0.02; the wavelength units cancel.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 5

A line is at 510 nm instead of 500 nm. Calculate fractional wavelength shift. Show the reasoning, not just the conclusion.
Hints

Not every redshift alone determines a unique distance or cause.

Answer and explanation

(510−500)/500=0.02.

Problem 6

Why does a galaxy redshift–distance pattern support expansion more strongly than one spectrum?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

A systematic relationship across many galaxies tests a global model rather than one object's local motion.

Problem 7

A broad distance–redshift relation supports expansion with qualificationsA broad distance–redshift relation supports expansion with qualificationsNearby-model recession speed (km/s)0700140021002800010203040Illustrative distance (Mpc)A supplied slope 70 km/s per Mpc; real estimates have uncertainty.
Original deliberately supplied linear relation with slope 70 km/s/Mpc for model reading. It is not a current observed galaxy dataset or an exact cosmological conversion for all redshifts. The graph makes its distance unit and nearby-approximation limit explicit.
Interpret the supplied visual model. At 30 Mpc in the supplied relation, what speed is predicted, and could a local motion cause a deviation?
Answer and explanation

2100 km/s. Local gravitational motion and measurement/calibration differences can cause deviations from the broad relation; a nearby exception does not automatically refute the expansion model.

Read distance and speed as different axes. The straight line is an illustrative nearby approximation, not a route traveled by one galaxy over time.

2100 km/s. Local gravitational motion and measurement/calibration differences can cause deviations from the broad relation; a nearby exception does not automatically refute the expansion model.

Response criteria

  • Read the labeled quantities, reference frame or objects correctly.
  • Explain the relation between the visual evidence and the mechanism.
  • Keep the drawing’s stated assumptions and limits in the conclusion.

Problem 8

A 500 nm laboratory line is observed at 575 nm, and a 600 nm line at 690 nm. Calculate z for each. Explain what the agreement supports and what it does not uniquely establish.
Answer and explanation

Both give z = 0.15: 75/500 and 90/600. Agreement supports a common fractional shift of that identified pattern, while the physical cause, distance and full cosmological interpretation need context, calibration and additional evidence.

Compute (575−500)/500 = 0.15.

Compute (690−600)/600 = 0.15.

Interpret the common fractional shift without assigning a unique cause from arithmetic alone.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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